Document:

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                                                                    EXHIBIT 10.9

                              NxSTAGE MEDICAL, INC.
                            INVESTOR RIGHTS AGREEMENT

      THIS INVESTOR RIGHTS AGREEMENT (this "Agreement") is entered into as of
the 30th day of June, 1999, by and among NxSTAGE MEDICAL, INC. (formerly known
as Qb Medical, Inc.), a Delaware corporation (the "Company"), and the purchasers
of the Company's Series B Preferred Stock par value $0.001 per share (the
"Series B Preferred Stock") set forth on the List of Investors attached hereto
as Exhibit A (hereinafter, the "Investors" and each individually an "Investor").

                                    RECITALS

      WHEREAS, the Company and the Investors have entered into a Series B
Preferred Stock Purchase Agreement of even date herewith (the "Purchase
Agreement"), pursuant to which the Investors will purchase that number of shares
of Series B Preferred Stock set forth opposite its name on Exhibit A;

      WHEREAS, the Company proposes to issue and sell an aggregate of One
Million Eight Hundred Seventy-Five Thousand (1,875,000) shares of its Series B
Preferred Stock pursuant to the Purchase Agreement;

      WHEREAS, as a condition of entering into the Purchase Agreement, the
Investors have requested that the Company extend to them registration rights,
information rights and other rights as set forth below.

      NOW THEREFORE, in consideration of the mutual promises, representations,
warranties, covenants and conditions set forth in this Agreement and in the
Purchase Agreement, the parties mutually agree as follows:

1. GENERAL

      1.1 DEFINITIONS. As used in this Agreement the following terms shall have
the following respective meanings:

      "CONVERSION SHARES" means the shares of the Company's common stock, par
value $0.001 per share ("Common Stock") issued upon conversion of the Shares.

      "EQUITY SECURITIES" means (i) any common stock, preferred stock or other
security of the Company, (ii) any security convertible, with or without
consideration, into any common stock, preferred stock or other security
(including any option to purchase such a convertible security), (iii) any
security carrying any warrant or right to subscribe to or purchase any common
stock, preferred stock or other security or (iv) any such warrant or right.

      "EXCHANGE ACT" means the Securities Exchange Act of 1934, as amended.

      "FORM S-3" means such form under the Securities Act as in effect on the
date hereof or any successor registration form under the Securities Act
subsequently adopted by the SEC which

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permits inclusion or incorporation of substantial information by reference to
other documents filed by the Company with the SEC.

      "HOLDER" means any person owning of record Registrable Securities that
have not been sold to the public or any assignee of record of such Registrable
Securities in accordance with Section 2.10 hereof.

      "INITIAL OFFERING" means the Company's first firmly underwritten public
offering of its Common Stock registered under the Securities Act.

      "REGISTER," "REGISTERED," AND "REGISTRATION" refer to a registration
effected by preparing and filing a registration statement in compliance with the
Securities Act, and the declaration or ordering of effectiveness of such
registration statement or document.

      "REGISTRABLE SECURITIES" means (i) Common Stock of the Company issued or
issuable upon conversion of the Shares or upon conversion pursuant to the
Purchase Agreement of shares of the Company's Series A Preferred Stock held by
any of the Investors; and (ii) any Common Stock of the Company issued as (or
issuable upon the conversion or exercise of any warrant, right or other security
which is issued as) a dividend or other distribution with respect to, or in
exchange for or in replacement of, such above-described securities.
Notwithstanding the foregoing, Registrable Securities shall not include any
securities sold by a person to the public either pursuant to a registration
statement or Rule 144 or sold in a private transaction in which the transferor's
rights under Section 2 of this Agreement are not assigned.

      "REGISTRABLE SECURITIES THEN OUTSTANDING" shall mean the total number of
shares of the Company's Common Stock that are Registrable Securities and either
(1) are then issued and outstanding, or (2) are issuable pursuant to then
exercisable or convertible securities.

      "REGISTRATION EXPENSES" shall mean all expenses incurred by the Company in
complying with Sections 2.2, 2.3 and 2.4 hereof, including, without limitation,
all registration and filing fees, printing expenses, fees and disbursements of
counsel for the Company, reasonable fees and disbursements not to exceed Fifteen
Thousand Dollars ($15,000) of a single special counsel for the Holders, blue sky
fees and expenses and the expense of any special audits incident to or required
by any such registration (but excluding the compensation of regular employees of
the Company which shall be paid in any event by the Company).

      "SECURITIES ACT" shall mean the Securities Act of 1933, as amended.

      "SELLING EXPENSES" shall mean all underwriting discounts and selling
commissions applicable to the sate.

      "SEC" or "COMMISSION" means the United States Securities and Exchange
Commission.

      "SHARES" shall mean the Company's Series B Preferred Stock issued pursuant
to the purchase Agreement.

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2. REGISTRATION; RESTRICTIONS ON TRANSFER

      2.1 RESTRICTIONS ON TRANSFER.

            a. Each Holder agrees not to make any disposition of all or any
portion of the Shares or Registrable Securities unless and until:

                  i. There is then in effect a registration statement under the
Securities Act covering such proposed disposition and such disposition is made
in accordance with such registration statement; or

                  ii. (A) The transferee has agreed in writing to be bound by
this Section 2.1. (B) such Holder shall have notified the Company of the
proposed disposition and shall have furnished the Company with a detailed
statement of the circumstances surrounding the proposed disposition, and (C) if
reasonably requested by the Company, such Holder shall have furnished the
Company with an opinion of counsel, reasonably satisfactory to the Company, that
such disposition will not require registration of such shares under the
Securities Act. It is agreed that the Company will not require a Holder to
obtain an opinion of counsel to such Holder for transactions made pursuant to
Rule 144 except in unusual circumstances.

                  iii. Notwithstanding the provisions of paragraphs (i) and (ii)
above, no such registration statement or opinion of counsel shall be necessary
for a transfer by a Holder which is (A) a partnership to its partners or former
partners in accordance with partnership interests, (B) a corporation to its
shareholders in accordance with their interest in the corporation, (C) a limited
liability company to its members or former members in accordance with their
interest in the limited liability company, or (D) to the Holder's family member
or trust for the benefit of an individual Holder, provided the transferee will
be subject to the terms of this Section 2.1 to the same extent as if such
transferee were an original Holder hereunder.

            b. Each certificate representing Shares or Registrable Securities
shall (unless otherwise permitted by the provisions of the Agreement) be stamped
or otherwise imprinted with a legend substantially similar to the following (in
addition to any legend required under applicable state securities laws or as
provided elsewhere in this Agreement):

            THE SECURITIES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED UNDER THE
            SECURITIES ACT OF 1933 (THE "ACT") AND MAY NOT BE OFFERED, SOLD OR
            OTHERWISE TRANSFERRED, ASSIGNED, PLEDGED OR HYPOTHECATED UNLESS AND
            UNTIL REGISTERED UNDER THE ACT OR UNLESS THE COMPANY HAS RECEIVED AN
            OPINION OF COUNSEL SATISFACTORY TO THE COMPANY AND ITS COUNSEL THAT
            SUCH REGISTRATION IS NOT REQUIRED.

            c. The Company shall be obligated to reissue promptly unlegended
certificates at the request of any Holder thereof if the Holder shall have
obtained an opinion of counsel (which counsel may be counsel to the Company)
reasonably acceptable to the Company

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to the effect that the securities proposed to be disposed of may lawfully be so
disposed of without registration, qualification or legend.

            d. Any legend endorsed on an instrument pursuant to applicable state
securities laws and the stop-transfer instructions with respect to such
securities shall be removed upon receipt by the Company of an order of the
appropriate blue sky authority authorizing such removal.

      2.2 DEMAND REGISTRATION.

            a. Subject to the conditions of this Section 2.2, if the Company
shall receive a written request from the Holders of more than thirty percent
(30%) of the Shares or the Registrable Securities (or a combination of such
Shares and Registrable Securities) then outstanding (the "Initiating Holders")
that the Company file a registration statement under the Securities Act covering
the registration of Registrable Securities having a reasonably anticipated
aggregate offering price to the public, net of underwriting discounts and
commissions, in excess of $5,000,000 (a "Qualified Public Offering"), then the
Company shall, within thirty (30) days of the receipt thereof, give written
notice of such request to all Holders, and subject to the limitations of this
Section 2.2, effect, as soon as practicable, the registration under the
Securities Act of all Registrable Securities that the Holders request to be
registered.

            b. If the Initiating Holders intend to distribute the Registrable
Securities covered by their request by means of an underwriting, they shall so
advise the Company as a part of their request made pursuant to this Section 2.2
and the Company shall include such information in the written notice referred to
in this Section 2.2. In such event, the right of any Holder to include its
Registrable Securities in such registration shall be conditioned upon such
Holder's participation in such underwriting and the inclusion of such Holder's
Registrable Securities in the underwriting (unless otherwise mutually agreed by
a majority in interest of the Initiating Holders and such Holder) to the extent
provided herein. All Holders proposing to distribute their securities through
such underwriting shall enter into an underwriting agreement in customary form
with the underwriter or underwriters selected for such underwriting by a
majority in interest of the Initiating Holders (which underwriter or
underwriters shall be reasonably acceptable to the Company). Notwithstanding any
other provision of this Section 2.2, if the underwriter advises the Company that
marketing factors require a limitation of the number of securities to be
underwritten (including Registrable Securities) then the Company shall so advise
all Holders of Registrable Securities which would otherwise be underwritten
pursuant hereto, and the number of shares that May be included in the
underwriting shall be allocated to the Holders of such Registrable Securities on
a pro rata basis based on the number of Registrable Securities held by all
Holders (including the initiating Holders). Any Registrable Securities excluded
or withdrawn from such underwriting shall be withdrawn from the registration.

            c. The Company shall not be required to effect a registration
pursuant to this Section 2.2:

                  i. After the Company has effected two (2) registrations
pursuant to this Section 2.2, and such registrations have been declared or
ordered effective; or

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                  ii. During the period starting with the date of filing of, and
ending on the date one hundred eighty (180) days following the effective date of
the registration statement pertaining to the Initial Offering, provided that the
Company is making reasonable and good faith effects to cause such registration
statement to become effective; or

                  iii. If within thirty (30) days of receipt of a written
request from Initiating Holders pursuant to Section 2.2 the Company gives notice
to the Holders of the Company's intention to make its Initial Offering within
ninety (90) days; or

                  iv. If the Company shall furnish to Holders requesting a
registration statement pursuant to this Section 2.2, a certificate signed by the
Chairman of the Board of Directors of the Company (the "Board") stating that in
the good faith judgment of the Board, it would be seriously detrimental to the
Company and its shareholders for such registration statement to be effected at
such time, in which event the Company shall have the right to defer such filing
for a period of not more than ninety (90) days after receipt of the request of
the Initiating Holders; provided, however, that such right to delay a request
shall be exercised by the Company no more than once in any one-year period.

      2.3 PIGGYBACK REGISTRATIONS. The Company shall notify all Holders of
Registrable Securities in writing at least thirty (30) days prior to the filing
of any registration statement under the Securities Act for purposes of a public
offering of securities of the Company (including, but not limited to,
registration statements relating to secondary offerings of securities of the
Company, but excluding registration statements relating to employee benefit
plans or with respect to corporate reorganizations or other transactions under
Rule 145 of the Securities Act) and will afford each such Holder an opportunity
to include in such registration statement all or part of such Registrable
Securities held by such Holder. Each Holder desiring to include in any such
registration statement all or any part of the Registrable Securities held by it
shall, within fifteen (15) days after the above-described notice from the
Company, so notify the Company in writing. Such notice shall state the intended
method of disposition of the Registrable Securities by such Holder. If a Holder
decides not to include all of its Registrable Securities in any registration
statement thereafter filed by the Company, such Holder shall nevertheless
continue to have the right to include any Registrable Securities in any
subsequent such registration statement or registration statements as may be
filed by the Company with respect to offerings of its securities, all upon the
terms and conditions set forth herein.

            a. If the registration statement under which the Company gives
notice under this Section 2.3 is for an underwritten offering, the Company shall
so advise the Holders of Registrable Securities. In such event, the right of any
such Holder to be included in a registration pursuant to this Section 2.3 shall
be conditioned upon such Holder's participation in such underwriting and the
inclusion of such Holder's Registrable Securities in the underwriting to the
extent provided herein. All Holders proposing to distribute their Registrable
Securities through such underwriting shall enter into an underwriting agreement
in customary form with the underwriter or underwriters selected for such
underwriting by the Company. Notwithstanding any other provision of this
Agreement, if the underwriter determines in good faith that marketing factors
require a limitation of the number of shares to be underwritten, the number of
shares that may be included in the underwriting shall be allocated, first, to
the Company; second, to the Holders on a pro rata basis based on the total
number of Registrable Securities held by the

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Holders; and third, to any shareholder of the Company (other than a Holder) on a
pro rata basis. No such reduction shall reduce the securities being offered by
the Company for its own account to be included in the registration and
underwriting, and in no event shall the amount of securities of the selling
Holders included in the registration be reduced below twenty percent (20%) of
the total amount of securities requested by the selling Holders to be included
in such registration, unless such offering is the Initial Offering and such
registration does not include shares of any other selling shareholders, in which
event any or all of the Registrable Securities of the Holders may be excluded in
accordance with the immediately preceding sentence. In no event will shares of
any other selling shareholder be included in such registration which would
reduce the number of shares which may be included by Holders without the written
consent of Holders of not less than two-thirds (66 2/3%) of the Registrable
Securities proposed to be sold in the offering.

            b. The Company shall have the right to terminate or withdraw any
registration initiated or withdraw any registration initiated by it under this
Section 2.3 prior to the effectiveness such registration whether or not any
Holder has elected to include securities in such registration. The Registration
Expenses of such withdrawn registration shall be borne by the Company in
accordance with Section 2.5 hereof.

      2.4 FORM S-3 REGISTRATION. In case the Company shall receive from any
Holder or Holders of Registrable Securities a written request or requests that
the Company effect a registration on Form S-3 or any similar short-form
registration statement and any related qualification or compliance with respect
to all or a part of the Registrable Securities owned by such Holder or Holders,
the Company will:

            a. Promptly give written notice of the proposed registration, and
any related qualification or compliance, to all other Holders of Registrable
Securities; and

            b. As soon as practicable, effect such registration and all such
qualifications and compliances as may be so requested and as would permit or
facilitate the sale and distribution of all or such portion of such Holder's or
Holders' Registrable Securities as are specified in such request, together with
all or such portion of the Registrable Securities of any other Holder or Holders
joining in such request as are specified in a written request given within
fifteen (15) days after receipt of such written notice from the Company;
provided, however, that the Company shall not be obligated to effect any such
registration, qualification or compliance pursuant to this Section 2.4:

                  i. If Form S-3 is not available for such offering by the
Holders, or

                  ii. If the Holders, together with the holders of any other
securities of the Company entitled to inclusion in such registration, propose to
sell Registrable Securities and such other securities (if any) at an aggregate
price to the public of less than five hundred thousand dollars ($500,000), or

                  iii. If the Company shall furnish to the Holders a certificate
signed by the Chairman of the, Board of Directors of the Company stating that in
the good faith judgment of the, Board of Directors of the Company, it would be
seriously detrimental to the Company and its shareholders for such Form S-3
Registration to be effected at such time, in which event the

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Company shall have the right to defer the filing of the Form S-3 registration
statement for a period of not more than ninety (90) days after receipt of the
request of the Holder or Holders under this Section 2.4; provided, that such
right to delay a request shall be exercised by the Company nor more than twice
in any one-year period, or

                  iv. If the Company has, already effected two (2) registrations
on Form S-3 for the Holders pursuant to this Section 2.4, or

                  v. In any particular jurisdiction in which the Company would
be required to qualify to do business or to execute a general consent to service
of process in effecting such registration, qualification or compliance.

            c. Subject to the foregoing, the Company shall file a Form S-3
registration statement covering the Registrable Securities and other securities
so requested to be registered as soon as practicable after receipt of the
request or requests of the Holders. All such Registration Expenses incurred in
connection with registrations requested pursuant to this Section 2.4 after the
first two (2) registrations shall be paid by the selling Holders pro rata in
proportion to the number of shares sold by each.

      2.5 EXPENSES OF REGISTRATION. Except as specifically provided herein, all
Registration Expenses incurred in connection with any registration,
qualification or compliance pursuant to Section 2.2 or any registration under
Section 2.3 or Section 2.4 herein shall be borne by the Company. All Selling
Expenses incurred in connection with any registrations hereunder, shall be borne
by the holders of the securities so registered pro rata on the basis of the
number of shares so registered. The Company shall not, however, be required to
pay for expenses of any registration proceeding begun pursuant to Section 2.2 or
2.4, the request of which has been subsequently withdrawn by the Initiating
Holders unless (a) the withdrawal is based upon material adverse information
concerning the Company of which the Initiating Holders were not aware at the
time of such request or (b) the Holders of a majority of Registrable Securities
agree to forfeit their right to one requested registration pursuant to Section
2.2 or Section 2.4, as applicable, in which event such right shall be forfeited
by all Holders). If the Holders are required to pay the Registration Expenses,
such expenses shall be borne by the holders of securities (including Registrable
Securities) requesting such registration in proportion to the number of shares
for which registration was requested. If the Company is required to pay the
Registration Expenses of a withdrawn offering pursuant to clause (a) above, then
the Holders shall not forfeit their rights pursuant to Section 2.2 or Section
2.4 to a demand registration.

      2.6 OBLIGATIONS OF THE COMPANY. Whenever required to effect the
registration of any Registrable Securities, the Company shall, as expeditiously
as reasonably possible:

            a. Prepare and file with the SEC a registration statement with
respect to such Registrable Securities and use all reasonable efforts to cause
such registration statement to become effective, and, upon the request of the
Holders of a majority of the Registrable Securities registered thereunder, keep
such registration statement effective for up to ninety (90) days or, if earlier,
until the Holder or Holders have completed the distribution related thereto.

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            b. Prepare and file with the SEC such amendments and supplements to
such registration statement and the prospectus used in connection with such
registration statement as may be necessary to comply with the provisions of the
Securities Act with respect to the disposition of all securities covered by such
registration statement.

            c. Furnish to the Holders such number of copies of a prospectus,
including a preliminary prospectus, in conformity with the requirements of the
Securities Act, and such other documents as they may reasonably request in order
to facilitate the disposition of Registrable Securities owned by them.

            d. Use all reasonable efforts to register and qualify the securities
covered by such registration statement under such other securities or Blue Sky
laws of such jurisdictions as shall be reasonably requested by the Holders,
provided that the Company shall not be required in connection therewith or as a
condition thereto to qualify to do business or to file a general consent to
service of process in any such states or jurisdictions.

            e. In the event of any underwritten public offering, enter into and
perform its obligations under an underwriting agreement, in usual and customary
form, with the managing underwriter(s) of such offering. Each Holder
participating in such underwriting shall also enter into and perform its
obligations under such an agreement.

            f. Notify each Holder of Registrable Securities covered by such
registration statement at any time when a prospectus relating thereto is
required to be delivered under the Securities Act of the happening of any event
as a result of which the prospectus included in such registration statement, as
then in effect, includes an untrue statement of a material fact or omits to
state a material fact required to be stated therein or necessary to make the
statements therein not misleading in the light of the circumstances then
existing.

            g. Furnish, at the request of a majority of the Holders
participating in the registration, on the date that such Registrable Securities
are delivered to the underwriters for sale, if such securities are being sold
through underwriters, or, if such securities are not being sold through
underwriters, on the date that the registration statement with respect to such
securities becomes effective, (i) an opinion, dated as of such date, of the
counsel representing the Company for the purposes of such registration, in form
and substance as is customarily given to underwriters in an underwritten public
offering and reasonably satisfactory to a majority in interest of the Holders
requesting registration, addressed to the underwriters, if any, and to the
Holders requesting registration of Registrable Securities and (ii) a letter
dated as of such date, from the independent certified public accountants of the
Company, in form and substance as is customarily given by independent certified
publics accountants to underwriters in an underwritten public offering and
reasonably satisfactory to a majority in interest of the Holders requesting
registration, addressed to the underwriters, if any, and if permitted by
applicable accounting standards, to the Holders requesting registration of
Registrable Securities.

      2.7 TERMINATION OF REGISTRATION RIGHTS. All registration rights granted
under this Section 2 shall terminate and be of no further force and effect seven
(7) years after the date of the Company's Initial Offering. In addition, a
Holder's registration rights shall expire if (i) the Company has completed its
Initial Offering and is subject to the provisions of the Exchange Act

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and (ii) all Registrable Securities held by and issuable to such Holder may be
sold under Rule 144 during any ninety (90) day period.

      2.8 DELAY OF REGISTRATION; FURNISHING INFORMATION.

            a. No Holder shall have any right to obtain or seek an injunction
restraining or otherwise delaying any such registration as the result of any
controversy that might arise with respect to the interpretation or
implementation of this Section 2.

            b. It shall be a condition precedent to the obligations of the
Company to take any action pursuant to Section 2.2, 2.3 or 2.4 that the selling
Holders shall furnish to the Company such information regarding themselves, the
Registrable Securities held by them and the intended method of disposition of
such securities as shall be required to effect the registration of their
Registrable Securities.

      2.9 INDEMNIFICATION. In the event any Registrable Securities are included
in a registration statement under Sections 2.2, 2.3 or 2.4:

            a. To the extent permitted by law, the Company will indemnify and
hold harmless each Holder, the partners, officers, directors and legal counsel
of each Holder, any underwriter (as defined in the Securities Act) for such
Holder and each person, if any, who controls such Holder or underwriter within
the meaning of the Securities Act or the Exchange Act, against any losses,
claims, damages, or liabilities (joint or several) to which they may become
subject under the Securities Act, the Exchange Act or other federal or state
law, insofar as such losses, claims, damages or liabilities (or actions in
respect thereof) arise out of or are based upon any of the following statements,
omissions or violations (collectively a "Violation") by the Company: (i) any
untrue statement or alleged untrue statement of a material fact contained in
such registration statement, including any preliminary prospectus or final
prospectus contained therein or any amendments or supplements thereto, (ii) the
omission or alleged omission to state therein material fact required to be
stated therein, or necessary to make the statements therein not misleading, or
(iii) any violation or alleged violation by the Company of the Securities Act,
the Exchange Act, any state securities law or any rule or regulation promulgated
under the Securities Act, the Exchange Act or any state securities law in
connection with the offering covered by such registration statement; and the
Company will reimburse each such Holder, partner, officer or director,
underwriter or controlling person for any legal or other expenses reasonably
incurred by them in connection with investigating or defending any such loss,
claim, damage, liability or action; provided however, that the indemnity
agreement contained in this Section 2.9(a) shall not apply to amounts paid in
settlement of any such loss, claim, damage, liability or action if such
settlement is effected without the consent of the Company, which consent shall
not be unreasonably withheld, nor shall the Company be liable in any such case
for any such loss, claim, damage, liability or action to the extent that it
arises out of or is based upon a Violation which occurs in reliance upon and in
conformity with written information furnished expressly for use in connection
with such registration by such Holder, partner, officer, director, underwriter
or controlling person of such Holder.

            b. To the extent permitted by law, each Holder will, if Registrable
Securities held by such Holder are included in the securities as to which such
registration qualifications or

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compliance is being effected, indemnify and hold harmless the Company, each of
its directors, its officers, and legal counsel and each person, if any, who
controls the Company within the meaning of the Securities Act, any underwriter
and any other Holder selling securities under such registration statement or any
of such other Holder's partners, directors or officers or any person who
controls such Holder, against any losses, claims, damages or liabilities (joint
or several) to which the Company or any such director, officer, controlling
person, underwriter or other such Holder, or partner, director, officer or
controlling person of such other Holder may become subject under the Securities
Act, the Exchange Act or other federal or state law, insofar as such losses,
claims, damages or liabilities (or actions in respect thereto) arise out of or
are based upon any Violation, in each case to the extent (and only to the
extent) that such Violation occurs in reliance upon and in conformity with
written information furnished by such Holder under an instrument duly executed
by such Holder and stated to be specifically for use in connection with such
registration; and each such Holder will reimburse any legal or other expenses
reasonably incurred by the Company or any such director, officer, controlling
person, underwriter or other Holder, or partner, officer, director or
controlling person of such other Holder in connection with investigating or
defending any such loss, claim, damage, liability or action; provided, however,
that the indemnity agreement contained in this Section 2.9(b) shall not apply to
amounts paid in settlement of any such loss, claim, damage, liability or action
if such settlement is effected without the consent of the Holder, which consent
shall not be unreasonably withheld; and provided further, that in no event shall
any indemnity under this Section 2.9 exceed the proceeds from the offering
received by such Holder.

            c. Promptly after receipt by an indemnified party under this Section
2.9 of notice of the commencement of any action (including any governmental
action), such indemnified party will, if a claim in respect thereof is to be
made against any indemnifying party under this Section 2.9, deliver to the
indemnifying party a written notice of the commencement thereof and the
indemnifying party shall have the right to participate in, and, to the extent
the indemnifying party so desires, jointly with any other indemnifying party
similarly noticed, to assume the defense thereof with counsel mutually
satisfactory to the parties; provided, however, that an indemnified party shall
have the right to retain its own counsel, with the fees and expenses to be paid
by the indemnifying party, if representation of such indemnified party by the
counsel retained by the indemnifying party would be inappropriate due to actual
or potential differing interests between such indemnified party and any other
party represented by such counsel in such proceeding. The failure to deliver
written notice to the indemnifying party within a reasonable time of the
commencement of any such action, if materially prejudicial to its ability to
defend such action, shall relieve such indemnifying party of any liability to
the indemnified party under this Section 2.9, but the omission so to deliver
written notice to the indemnifying party will not relieve it of any liability
that it may have to any indemnified party otherwise than under this Section 2.9.

            d. If the indemnification provided for in this Section 2.9 is held
by a court of competent jurisdiction to be unavailable to an indemnified party
with respect to any losses, claims, damages or liabilities referred to herein,
the indemnifying party, in lieu of indemnifying such indemnified party
thereunder, shall to the extent permitted by applicable law contribute to the
amount paid or payable by such indemnified party as a result of such loss,
claim, damage or liability in such proportion as is appropriate to reflect the
relative fault of the indemnifying party on the one hand and of the indemnified
party on the other in connection with the Violation(s)

                                      -10-
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that resulted in such loss, claim, damage or liability, as well as any other
relevant equitable considerations. The relative fault of the indemnifying party
and of the indemnified party shall be determined by a court of law by reference
to, among other things, whether the untrue or alleged true statement of a
material fact or the omission to state a material fact relates to information
supplied by the indemnifying party or by the indemnified party and the parties'
relative intent, knowledge, access to information and opportunity to correct or
prevent such statement or omission; provided, that in no event shall any
contribution by a Holder hereunder exceed the proceeds from the offering
received by such Holder.

            e. The obligations of the Company and Holders under this Section 2.9
shall survive completion of any offering of Registrable Securities in a
registration statement. No indemnifying party, in the defense of any such claim
or litigation, shall, except with the consent of each indemnified party, consent
to entry of any judgment or enter into any settlement which does not include as
an unconditional term thereof the giving by the claimant or plaintiff to such
indemnified party of a release from all liability in respect to such claim or
litigation. In the event any offering of Registrable Securities is underwritten,
and the underwriting agreement provides for indemnification and/or contribution
by the Company and the Holders offering securities thereunder, the
indemnification and/or contribution obligations of the Company and the Holders
hereunder shall in no event exceed the obligations of the parties set forth in
such underwriting agreement.

      2.10 ASSIGNMENT OF REGISTRATION RIGHTS. The rights to cause the Company to
register Registrable Securities pursuant to this Section 2 may be assigned by a
Holder to a transferee or assignee of Registrable Securities which (i) is a
subsidiary, parent, general partner, limited partner or retired partner of a
Holder. (ii) is a Holder's family member or trust for the benefit of an
individual Holder, or (iii) acquires at least one hundred thousand (100,000)
shares of Registrable Securities (as presently constituted and as adjusted for
stock splits, reverse stock splits, stock dividends, recapitalizations,
combinations and the like); provided, however, (A) the transferor shall, within
ten (10) days after such transfer, furnish to the Company written notice of the
name and address of such transferee or assignee and the securities with respect
to which such registration rights are being assigned and (B) such transferee
shall agree to be subject to all restrictions set forth in this Agreement.

      2.11 AMENDMENT OF REGISTRATION RIGHTS. Any provision of this Section 2 may
be amended and the observance thereof may be waived (either generally or in a
particular instance and either retroactively or prospectively), only with the
written consent of the Company and the Holders of at least sixty-six and
two-thirds percent (66 2/3 %) of the Registrable Securities. Any amendment or
waiver effected in accordance with this Section 2.11 shall be binding upon each
Holder and the Company. By acceptance of any benefits under this Section 2,
Holders of Registrable Securities hereby agree to be bound by the provisions
hereunder.

      2.12 LIMITATION ON SUBSEQUENT REGISTRATION RIGHTS. After the date of this
Agreement, the Company shall not, without the prior written consent of the
Holders of sixty-six and two-thirds percent (66 2/3%) of the Registrable
Securities, enter into any agreement with any holder or prospective holder of
any securities of the Company that would grant such holder registration rights
senior to those granted to the Holders hereunder.

                                      -11-
<PAGE>

      2.13 "MARKET STAND-OFF" AGREEMENT. If requested by the Company as the
representative of the underwriters of Common Stock (or other securities) of the
Company, each Holder shall not sell or otherwise transfer or dispose of any
Shares Common Stock (or other securities) of the Company held by such each
Holder (other than those included in the registration) for a period specified by
the representative of the underwriters not to exceed one hundred eighty (180)
days following the effective date of a registration statement of the Company
filed under the Securities Act, provided that all officers and directors of the
Company and holders of at least one percent (1%) of the Company's voting
securities enter into similar agreements.

      The obligations described in this Section 2.13 shall not apply to a
registration relating solely to employee benefit plans on Form S-3 or Form S-8
or similar forms that may be promulgated in the future, or a registration
relating solely to a Commission Rule 145 transaction on Form S-4 or similar
forms that may be promulgated in the future. The Company may impose
stop-transfer instructions with respect to the shares of Common Stock (or other
securities) subject to the foregoing restriction until the end of said one
hundred eighty (180) day period.

      2.14 RULE 144 REPORTING. With a view to making available to the Holders
the benefits of certain rules and regulations of the SEC which may permit the
sale of the Registrable Securities to the public without registration, the
Company agrees to use its best efforts to:

            a. Make and keep public information available, as those terms are
understood and defined in SEC Rule 144 or any similar or analogous rule
promulgated under the Securities Act, at all times after the effective date of
the first registration filed by the Company for an offering of its securities to
the general public;

            b. File with the SEC, in a timely manner, all reports and other
documents required of the Company under the Exchange Act;

            c. So long as a Holder owns any Registrable Securities, furnish to
such Holder forthwith upon request a written statement by the Company as to its
compliance with the reporting requirements of said Rule 144 of the Securities
Act and of the Exchange Act (at any time after it has become subject to such
reporting requirements), a copy of the most recent annual or quarterly report of
the Company, and such other reports and documents as a Holder may reasonably
request in availing itself of any rule or regulation of the SEC allowing it to
sell any such securities without registration.

3. COVENANTS OF THE COMPANY

      3.1 BASIC FINANCIAL INFORMATION AND REPORTING.

            a. The Company will maintain true books and records of account in
which full and correct entries will be made of all its business transactions
pursuant to a system of accounting established and administered in accordance
with generally accepted accounting principles consistently applied, and will set
aside on its books all such proper accruals and reserves as shall be required
under generally accepted accounting principles consistently applied.

                                      -12-
<PAGE>

            b. As soon as practicable after the end of each fiscal year of the
Company, and in any event within ninety (90) days thereafter, the Company will
furnish each Investor a consolidated balance sheet of the Company, as at the end
of such fiscal year, and a consolidated statement of income and a consolidated
statement of cash flows of the Company, for such year all prepared in accordance
with generally accepted accounting principles consistently applied and setting
forth in each case in comparative form the figures for the previous fiscal year,
all in reasonable detail. Such financial statements shall be accompanied by a
report and opinion thereon by independent public accountants of national
standing selected by the Company's Board of Directors.

            c. The Company will furnish each Investor, as soon as practicable
after the end of the first, second and third quarterly accounting periods in
each fiscal year of the Company, and any event within forty-five (45) days
thereafter, a consolidated balance sheet of the Company as of the end of each
such quarterly period, and a consolidated statement of income and a consolidated
statement of cash flows of the Company for such period and for the current
fiscal year to date, prepared in accordance with generally accepted accounting
principles, with the exception that no notes need be attached to such statements
and year-end audit adjustments may not have been made.

            d. So long as an Investor (with its affiliates) shall own not less
than one hundred thousand (100,000) shares of Registrable Securities (as
presently constituted and subject to subsequent adjustment for stock splits,
reverse stock splits, stock dividends, recapitalizations, combinations and the
like) (a "Major Investor"), the Company will furnish each such Major Investor
(i) at least thirty (30) days prior to the beginning of each fiscal year an
annual budget and operating plan for such fiscal year (and as soon as available,
any subsequent revisions thereto); and (ii) as soon as practicable after the end
of each month, and in any event within twenty (20) days thereafter, a
consolidated balance sheet of the Company as of the end of each such month, and
a consolidated statement of income and a consolidated statement of cash flows of
the Company for such month and for the current fiscal year to date, including a
comparison to plan figures for such period, prepared in accordance with
generally accepted accounting principles consistently applied, with the
exception that no notes need be attached to such statements and year-end audit
adjustments may not have been made.

      3.2 INSPECTION RIGHTS. Each Major Investor shall have the right to visit
and inspect any of the properties of the Company or any of its subsidiaries, and
to discuss the affairs, finances and accounts of the Company or any of its
subsidiaries with its officers, and to review such information as is reasonably
requested all at such reasonable times and as often as may be reasonably
requested; provided, however, that the Company shall not be obligated under this
Section 12 with respect to a competitor of the Company or with respect to
information which the Board of Directors determines in good faith is
confidential and should not, therefore, be disclosed.

      3.3 CONFIDENTIALITY OF RECORDS.

            a. Each Investor agrees not to use Confidential Information (as
hereinafter defined) of the Company for its own use or for any purpose except to
evaluate and enforce its equity investment in the Company. Each Investor shall
undertake to treat such Confidential

                                      -13-
<PAGE>

Information in a manner consistent with the treatment of its own information of
such proprietary nature and agrees that it shall protect the confidentiality of
and use reasonable best efforts to prevent disclosure of the Confidential
Information to prevent it from falling into the public domain or the possession
of unauthorized persons. Each transferee of any Investor who receives
Confidential Information shall agree to be bound by such provisions. For
purposes of this Section, "Confidential Information" means any information,
technical data, or know-how, including, but not limited to, the Company's
research, products, software, services, development, inventions, processes,
designs, drawings, engineering, marketing, or finances, disclosed by the Company
either directly or indirectly in writing, orally or by drawings or inspection of
parts or equipment.

            b. Confidential Information does not include information, technical
data or know-how which (i) is in the Investor's possession at the time of
disclosure as shown by Investor's files and records immediately prior to the
time of disclosure; (ii) before or after it has been disclosed to the Investor,
it is part of the public knowledge or literature, not as a result of any action
or inaction of the Investor; or (iii) is disclosed to an Investor on a
non-confidential basis by third party having a legal right to such information,
(iv) is reasonably demonstrated by Investor have been independently developed by
Investor, or (v) is approved for release by written authorization of Company.

            c. The provisions of this Section shall not apply (i) to the extent
that an Investor is required to disclose Confidential Information pursuant to
any law, statue, rule or regulation or any order of any court or jurisdiction
process or pursuant to any direction, request or requirement (whether or not
having the force of law but if not having the force of law being of a type with
which institutional investors in the relevant jurisdiction are accustomed to
comply) of any self-regulating organization or any governmental, fiscal,
monetary or other authority provided that the Company is given notice and an
opportunity to restrict reasonably the scope of such disclosure; (ii) to the
disclosure of Confidential Information to an Investor's employees, counsel,
accountants or other professional advisors provided such recipients also are
bound by obligations of confidentiality; (iii) to the extent that an Investor
needs to disclose Confidential Information for the protection of any of such
Investor's rights or interest against the Company, whether under this Agreement
or otherwise; or (iv) with the prior written consent of the Company, which
consent shall not be unreasonably withheld, to the disclosure of Confidential
Information to a prospective transferee of securities which agrees in writing to
be bound by the provisions of this Section in connection with the receipt of
such Confidential Information.

            d. The Company covenants and agrees that, promptly after the
issuance of the Series B Preferred Stock pursuant to the Purchase Agreement, it
shall adopt a confidentiality reasonably acceptable to the Investors, and that
it will at all times use reasonable efforts to stamp or otherwise conspicuously
mark written and other tangible Confidential Information "Confidential" or
"Proprietary" or, if and to the extent that Confidential Information is
disclosed orally, to promptly confirm in writing that such information is
Confidential Information.

      3.4 RESERVATION OF COMMON STOCK. The Company will at all times reserve and
keep available, solely for issuance and delivery upon the conversion of the
Shares, all Common Stock issuable from time to time upon such conversion.

                                      -14-
<PAGE>

      3.5 STOCK VESTING. Unless otherwise approved by seventy-five percent (75
%) of the of Directors, with all Directors voting:

            a. The Common Stock to be granted and issued to Directors pursuant
to Section 3.9 hereof shall vest at the rate of one third (1/3rd) of the stock
so granted to a Director at the end of each year for a period of three (3) years
commencing upon the date on which such Director is elected and qualified to
serve on the Board.

            b. All other stock options and other stock equivalents issued after
the date of Agreement to employees, officers, directors, consultants and other
service providers shall be subject to vesting as follows: (i) twenty-five
percent (25 %) of such stock shall vest at the end of the first year following
the earlier of the date of issuance or such person's services commencement date
with the company, and (ii) seventy-five percent (75 a) of such stock shall vest
at the rate of one thirty-sixth (1/36th) per month over the remaining three (3)
years. With respect to any shares of stock purchased by any such person, the
Company's repurchase option shall provide that upon such person's termination of
employment or service with the Company, with or without cause, the Company or
its assignee (to the extent permissible under applicable securities laws and
other laws) shall have the option to purchase at cost any unvested shares of
stock held by such person.

      3.6 KEY PERSON INSURANCE. Subject to the approval of the Board, the
Company will use its best efforts to obtain and maintain in full force and
effect term life insurance in the amount of one million ($1,000,000) dollars on
the life of the Chief Executive Officer of the Company naming the Company as
beneficiary.

      3.7 PROPRIETARY INFORMATION AND INVENTIONS AGREEMENT. The Company shall
require all employees to execute and deliver a Proprietary Information and
Inventions Agreement substantially in the form attached to the Purchase
Agreement.

      3.8 APPROVAL OF RELATED PARTY AGREEMENTS. The Company shall not without
the approval of a majority of the Board of Directors, with all non-interested
Directors voting, authorize or enter into any transactions with any director or
management employee, or such director's or employee's immediate family.

      3.9 DIRECTORS' EXPENSES. The Company shall reimburse all members of the
Company's Board of Directors for all reasonable and necessary costs and expenses
incurred by such members in connection with the performance of their duties as a
Director. In addition, as compensation for the performance of their duties as a
Director, the Company shall grant to each member Fifteen Thousand (15,000)
shares of the Company's Common Stock out of the pool of Common Stock reserved
for issuance to the Company's employees, consultants and directors, which shares
shall vest at the rate of one-third (1/3rd) per year at the end of each year
over a three (3) year period commencing upon the date such member is elected and
qualified as a Director.

      3.10 DIRECTORS' INDEMNIFICATION. The Company shall enter into and use its
best efforts to at all times maintain indemnification contracts, in a form
reasonably satisfactory to the

                                      -15-
<PAGE>

designees of the Series B Preferred Stock and their counsel, with each of its
directors to indemnify such directors to the maximum extent permissible under
Delaware law.

      3.11 REAL PROPERTY HOLDING CORPORATION. The Company covenants that it will
operate in a manner such that it will not become a "United States real property
holding corporation" ("USRPHC") as that term is defined in Section 897(c)(2) of
the Internal Revenue Code of 1986, as amended, and the regulations thereunder.
The Company agrees to make determinations as to its status as a USRPHC, and will
file statements concerning those determinations with the Internal Revenue
Service, in the manner and at the times required under Reg. Section 1.897-2(h),
or any supplementary or successor provision thereto. Within 30 days of a request
from an Investor or any of its partners, the Company will inform the requesting
party, in the manner set forth in Reg. Section 1.897 -2(h)(l)(iv) or any
supplementary or successor provision thereto, whether that party's interest in
the Company constitutes a United States real property interest (within the
meaning of Internal Revenue Code Section 897(c)(l) and the regulations
thereunder) and whether the Company has provided to the Internal Revenue Service
all required notices as to its USRPHC status.

      3.12 TERMINATION OF COVENANTS. All covenants of the Company contained in
Section 3 of this Agreement shall expire and terminate as to each Investor on
the effective date of the registration statement pertaining to the Initial
Offering.

4. RIGHTS OF FIRST REFUSAL

      4.1 SUBSEQUENT OFFERINGS. Each Investor shall have a right of first
refusal to purchase its pro rata share of all Equity Securities that the Company
may, from time to time, propose to sell and issue after the date of this
Agreement, other than the Equity Securities excluded by Section 4.6 hereof. Each
Investor's pro rata share is equal to the ratio of (A) the number of shares of
the Company's Common Stock, including Conversion Shares or shares issuable upon
conversion of the Shares, which such Investor is deemed to be a holder of
immediately prior to the issuance of such Equity Securities to (B) the total
number of shares of the Company's outstanding Common Stock (including all shares
of Common Stock issued or issuable upon conversion of the Shares) immediately
prior to the issuance of the Equity Securities and assuming exercise of all
outstanding options and warrants to purchase securities of the Company.

      4.2 EXERCISE OF RIGHTS. If the Company proposes to issue any Equity
Securities, it shall give each Investor written notice of its intention,
describing the Equity Securities, the price and the terms and conditions upon
which the Company proposes to issue the same. Each Investor shall have fifteen
(15) days from the giving of such notice to agree to purchase his, her or its
pro rata share of the Equity Securities for the price and upon the terms and
conditions specified in the notice by giving written notice to the Company and
stating therein the quantity of Equity Securities to be purchased.
Notwithstanding the foregoing, the Company shall not be required to offer or
sell such Equity Securities to any Investor who would cause the Company to be in
violation of applicable federal securities laws by virtue of such offer or sale.

      4.3 ISSUANCE OF EQUITY SECURITIES TO OTHER PERSONS. If not all of the
Investors elect to purchase their pro rata share of the Equity Securities, then
the Company shall promptly notify

                                      -16-
<PAGE>

in writing the Investors who do so elect and shall offer them the right to
acquire their pro rata shares of such unsubscribed shares. Such Investors shall
have five (5) days after receipt of such notice to notify the Company of his,
her or its election to purchase all or a portion thereof of the unsubscribed
shares. If the Investors so notified fail to exercise in full the rights of
first refusal, the Company shall have ninety (90) days thereafter to sell the
Equity Securities in respect of which the Investors' rights were not exercised,
at a price and upon general terms and conditions materially no more favorable to
the purchasers thereof than specified in the Company's notice to the Investors
pursuant to Section 4.2 hereof. If the Company has not sold such Equity
Securities within ninety (90) days of the notice provided pursuant to Section
4.2, the Company shall not thereafter issue or sell any Equity Securities,
without first offering such securities to the Investors in the manner provided
above.

      4.4 TERMINATION OF RIGHTS OF FIRST REFUSAL. The rights of first refusal
established by this Section 4 shall terminate upon the effective date of the
registration statement pertaining to a Qualified Public Offering.

      4.5 TRANSFER OF RIGHTS OF FIRST REFUSAL. The rights of first refusal of
each Investor under this Section 4 may be transferred to the same parties
subject to the same restrictions as those restrictions on transfer of
registration rights set forth in Section 2.10 hereof.

      4.6 EXCLUDED SECURITIES. The rights of first refusal established by this
Section 4 shall have no application to any of the following Equity Securities:

            a. Shares of Common Stock (and/or options, warrants or other Common
Stock purchase rights issued pursuant to such options, warrants or other rights)
issued or to be issued to employees, officers or directors of, or consultants or
advisors to the Company or any subsidiary, pursuant to stock purchase or stock
option plans or other arrangements that are approved by the Board of Directors;

            b. Stock issued pursuant to any rights or agreements outstanding as
of the date of this Agreement, options and warrants outstanding as of the date
of this Agreement; and stock issued pursuant to any such rights or agreements
granted after the date of this Agreement, provided that the rights of first
refusal established by this Section 4 applied with respect to the initial sale
or grant by the Company of such rights or agreements;

            c. Any Equity Securities issued for consideration other than cash
pursuant to a merger, consolidation, acquisition or similar business
combination;

            d. Shares of Common Stock issued in connection with any stock split,
stock dividend or recapitalization by the Company;

            e. Shares of Common Stock issued upon conversion of the Shares;

            f. Any Equity Securities issued pursuant to any equipment leasing
arrangement, or bank financing;

            g. Any Equity Securities that are issued by the Company pursuant to
a registration statement filed under the Securities Act; and

                                      -17-
<PAGE>

            h. Equity Securities issued in connection with strategic
transactions involving the Company and other entities, including (i) joint
ventures, manufacturing, marketing or distribution arrangements or (ii)
technology transfer or development arrangements; provided that such strategic
transactions and the issuance of shares therein, has been approved by not less
than seventy-five percent (75%) the Company's Board of Directors, with all
Directors voting.

5. MISCELLANEOUS

      5.1 GOVERNING LAW. This Agreement shall be governed by and construed under
the laws of the State of Delaware as applied to agreements among Delaware
residents entered into and to be performed entirely within Delaware.

      5.2 SURVIVAL. The representations, warranties, covenants, and agreements
made herein shall survive any investigation made by any Holder and the closing
of the transactions contemplated hereby. All statements as to factual matters
contained in any certificate or other instrument delivered by or on behalf of
the Company pursuant hereto in connection with the transactions contemplated
hereby shall be deemed to be representations and warranties by the Company
hereunder solely as of the date of such certificate or instrument.

      5.3 SUCCESSORS AND ASSIGNS. Except as otherwise expressly provided herein,
the provisions hereof shall inure to the benefit of, and be binding upon, the
successors, assigns, heirs, executors, and administrators of the parties hereto
and shall inure to the benefit of and be enforceable by each person who shall be
a holder of Registrable Securities from time to time; provided, however, that
prior to the receipt by the Company of adequate written notice of the transfer
of any Registrable Securities specifying the full name and address of the
transferee, the Company may deem and treat the person listed as the holder of
such shares in its records as the absolute owner and holder of such shares for
all purposes, including the payment of dividends or any redemption price.

      5.4 SEVERABILITY. In case any provision of the Agreement shall be invalid,
illegal, or unenforceable, the validity, legality, and enforceability of the
remaining provisions shall not in any way be affected or impaired thereby.

      5.5 AMENDMENT AND WAIVER.

            a. Except as otherwise expressly provided, this Agreement may be
amended or modified only upon the written consent of the Company and the holders
of at least two-thirds (66 2/3%) of the Registrable Securities.

            b. Except as otherwise expressly provided, the obligations of the
Company and the rights of the Holders under this Agreement may be waived only
with the written consent of the holders of at least two-thirds (66 2/3%) of the
Registrable Securities.

            c. Notwithstanding the foregoing, this Agreement may be amended with
only the written consent of the Company to include additional purchasers of
Shares as "Investors," "Holders" and parties hereto.

                                      -18-
<PAGE>

      5.6 DELAYS OR OMISSIONS. It is agreed that no delay or omission to
exercise any right, power, or remedy accruing to any Holder, upon any breach,
default or noncompliance of the Company under this Agreement shall impair any
such right, power, or remedy, nor shall it be construed to be a waiver of any
such breach, default or noncompliance, or any acquiescence therein, or of any
similar breach, default or noncompliance thereafter occurring. It is further
agreed that any waiver, permit, consent, or approval of any kind or character on
any Holder's part of any breach, default or noncompliance under the Agreement or
any waiver on such Holder's part of any provisions or conditions of this
Agreement must be in writing and shall be effective only to the extent
specifically set forth in such writing. All remedies, either under this
Agreement, by law, or otherwise afforded to Holders, shall be cumulative and not
alternative.

      5.7 NOTICES. All notices required or permitted hereunder shall be in
writing and shall be deemed effectively given: (i) upon personal delivery to the
party to be notified, (ii) when sent by confirmed telex or facsimile if sent
during normal business hours of the recipient; if not, then on the next business
day, (iii) five (5) days after having been sent by registered or certified mail,
return receipt requested, postage prepaid, or (iv) one (1) day after deposit
with a nationally recognized overnight courier, specifying next day delivery,
with written verification of receipt. All communications shall be sent to the
party to be notified at the address as set forth on the signature pages hereof
or Exhibit A hereto or at such other address as such party may designate ten
(10) days advance written notice to the other parties hereto.

      5.8 ATTORNEYS' FEES. In the event that any dispute among the parties to
this Agreement should result in litigation, the prevailing party in such dispute
shall be entitled to recover from the using party all fees, costs and expenses
of enforcing any right of such prevailing party under or with respect to this
Agreement, including without limitation, such reasonable fees and expenses of
attorneys and accountants, which shall include, without limitation, all fees,
costs and expenses of appeals.

      5.9 TITLES AND SUBTITLES. The titles of the sections and subsections of
this Agreement are for convenience of reference only and are not to be
considered in construing this Agreement.

      5.10 COUNTERPARTS. This Agreement may be executed in any number of
counterparts, each of which shall be an original, but all of which together
shall constitute one instrument.

                                      -19-
<PAGE>

      IN WITNESS WHEREOF, the parties hereto have executed this Investor Rights
Agreement as of the date first set forth above.

                                            COMPANY:

                                            NxSTAGE MEDICAL, INC.

                                            By: /s/ Jeffrey H. Burbank
                                                --------------------------
                                                Name: Jeffrey H. Burbank
                                                Title: President

                                      -20-
<PAGE>

                                            INVESTORS

                                            /s/ Jeffrey H. Burbank
                                            ---------------------------------
                                            Jeffrey H. Burbank

                                            /s/ David S. Utterberg
                                            ---------------------------------
                                            David S. Utterberg

                                            WPG ENTERPRISE FUND III, L.L.C.

                                            By: WPG VC Fund Adviser, L.L.C.,
                                                Fund Investment Advisory Member

                                            By: /s/ Jeani Delagardelle
                                                -----------------------------
                                                Name: Jeani Delagardelle
                                                Title: Managing Member

                                            WEISS, PECK & GREER VENTURE
                                            ASSOCIATES IV, L.L.C.

                                            By: WPG VC Fund Adviser, L.L.C.,
                                                Fund Investment Advisory Member

                                            By: /s/ Jeani Delagardelle
                                                -----------------------------
                                                Name: Jeani Delagardelle
                                                Title: Managing Member

                                            WEISS, PECK & GREER VENTURE
                                            ASSOCIATES IV CAYMAN, L.P.

                                            By: WPG Venture Advisers, Ltd.,
                                                Administrative General Partner

                                            By: /s/ Scott Dakers
                                                -----------------------------
                                                Name: Scott Dakers
                                                Title: Director

                                      -21-
<PAGE>

                                            ATLAS VENTURE FUND III, L.P.

                                            By: Atlas Venture Associates III,
                                                LLC
                                                Its General Partner

                                            By: /s/ [illegible]
                                                -----------------------------
                                                Name:
                                                Title:

                                            ATLAS VENTURE ENTREPRENEURS'
                                            FUND III, L.P.

                                            By: Atlas Venture Associates III,
                                                LLC
                                                Its General Partner

                                            By: /s/ [illegible]
                                                -----------------------------
                                                Name:
                                                Title:

                                      -22-
<PAGE>

                                            SPROUT CAPITAL VIII, L.P.

                                            By: DLJ CAPITAL CORP.
                                                Its Managing General Partner

                                            By: /s/ Philippe O. Chambon
                                                -----------------------------
                                                Name: Philippe O. Chambon
                                                Title: Attorney In Fact

                                            SPROUT VENTURE CAPITAL, L.P.

                                            By: DLJ CAPITAL CORP.
                                                Its Managing General Partner

                                            By: /s/ Philippe O. Chambon
                                                -----------------------------
                                                Name: Philippe O. Chambon
                                                Title: Attorney In Fact

                                            DLJ CAPITAL CORP.

                                            By: /s/ Philippe O. Chambon
                                                -----------------------------
                                                Name: Philippe O. Chambon
                                                Title: Attorney In Fact

                                            DLJ ESC II, L.P.

                                            By: DLJ LBO Plans Management
                                                Corporation
                                                Its General Partner

                                            By: /s/ Philippe O. Chambon
                                                -----------------------------
                                                Name: Philippe O. Chambon
                                                Title: Attorney In Fact

                                      -23-
<PAGE>

                                    EXHIBIT A

                                LIST OF INVESTORS

<TABLE>
<CAPTION>
                                                       Number        Aggregate
             Name and Address                        of Shares    Purchase Price
-------------------------------------------          ---------    ---------------
<S>                                                  <C>          <C>
Jeffrey H. Burbank                                      74,906    $    199,999.02
18 Sunrise Road
Boxford, Massachusetts 01921

Sprout Capital VIII, L.P.                              981,202    $  2,619,809.34
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

Sprout Venture Capital, L.P.                            58,872    $    157,188.24
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

DLJ Capital Corp.                                        3,273    $      8,738.91
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

DLJ ESC II, L.P.                                        85,383    $    227,972.61
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

WPG Enterprise Fund II, L.L.C                          132,054    $    352,584.18
Weiss, Peck & Greer, LLC
555 California Street, Suite 3130
San Francisco, California 94104

Weiss, Peck & Greer Venture                            151,057    $    403,322.19
  Associates IV, L.L.C
Weiss, Peck & Greer, LLC
555 California Street, Suite 3130
San Francisco, CA 94104
</TABLE>

                                      -24-
<PAGE>

<TABLE>
<CAPTION>
                                                       Number        Aggregate
             Name and Address                        of Shares    Purchase Price
-------------------------------------------          ---------    ---------------
<S>                                                  <C>          <C>
Weiss, Peck & Greer Venture Associates IV               19,003    $     50,738.01
  Cayman, L.P.
Bank American Trust & Banking Corporation
  (Cayman) Limited
P.O. Box 1092
BankAmerica House, Fort Street
George Town, Grand Cayman
Cayman Islands, British West Indies

David S. Utterberg                                     201,410    $    537,764.70
c/o Medisystems Corporation
1201 Third Avenue, 39th Floor
Seattle, Washington 98101-3016

Atlas Venture Fund III, L.P.                           164,268    $    438,595.56
222 Berkeley Street, Suite 1950
Boston, Massachusetts 02116

Atlas Venture Entrepreneurs' Fund III, L.P.              3,572    $      9,537.24
222 Berkeley Street, Suite 1950
Boston, Massachusetts 02116
                                                     ---------    ---------------
TOTAL                                                1,875,000    $  5,006,250.00
</TABLE>

                                      -25-
<PAGE>

                              NxSTAGE MEDICAL, INC.

                             FIRST AMENDMENT TO THE
                            INVESTOR RIGHTS AGREEMENT

      THIS FIRST AMENDMENT TO THE INVESTOR RIGHTS AGREEMENT (this "First
Amendment") is entered into as of the 24th day of January, 2000, by and among
NxSTAGE MEDICAL, INC., a Delaware corporation (the "Company"), and the
purchasers of the Company's Series B Preferred Stock par value $0.001 per share
(the "Series B Preferred Stock") and the Company's Series C Preferred Stock par
value $0.001 per share (the "Series C Preferred Stock") set forth on the
signature pages hereto (hereinafter, the "Investors" and each individually an
"Investor").

                                    RECITALS

      WHEREAS, the Investors purchased, in the aggregate, 1,875,000 shares of
Series B Preferred Stock pursuant to the Series B Preferred Stock Purchase
Agreement dated June 30, 1999 (the "Series B Purchase Agreement");

      WHEREAS, in connection with the acquisition of the Series B Preferred
Stock, the Company and the Investors entered into an Investor Rights Agreement
dated as of June 30, 1999 (the "Investor Rights Agreement");

      WHEREAS, the Company and the Investors have entered into a Series C
Preferred Stock Purchase Agreement of even date herewith (the "Series C Purchase
Agreement"), pursuant to which each of the Investors will purchase that number
of shares of Series C Preferred Stock set forth opposite his or its name on
Exhibit A totaling an aggregate of 1,151,632 shares of Series C Preferred Stock;
and

      WHEREAS, it is a condition to the closing of the sale of the Series C
Preferred Stock pursuant to the Series C Purchase Agreement, that the Company
grant to the Investors, with respect to the Series C Preferred Stock, those
registration rights, information rights and other rights as set forth in the
Investor Rights Agreement;

      NOW THEREFORE, in consideration of the mutual promises, representations,
warranties, covenants and conditions set forth herein and in the Series C
Purchase Agreement, the parties hereby agree as follows:

1. AMENDMENT TO THE INVESTOR RIGHTS AGREEMENT. In accordance with Section 5.5 of
the Investors Agreement, the Investors and the Company hereby agree to amend the
Investor Rights Agreement as follows:

      a. The definition of "Shares" set forth in Section 1 of the Investor
Rights Agreement is amended to read in its entirety as follows:

         "SHARES" means the Company's Series B Preferred Stock issued pursuant
to the Series B Purchase Agreement and the Company's Series C Preferred Stock
issued pursuant to the Series C Purchase Agreement.

<PAGE>

      b. All references to "Shares" throughout the Investor Rights Agreement
shall be read to refer to the amended definition of "Shares" as set forth above.

      c. Exhibit A to the Investor Rights Agreement is amended and restated as
set forth on the Exhibit A attached hereto.

      d. The remainder of the Investor Rights Agreement not amended hereby
remains unchanged and in full force and effect

2. MISCELLANEOUS

      2.1 GOVERNING LAW. This First Amendment is governed by and construed under
the laws of the Stare of Delaware as applied to agreements among Delaware
residents entered into and be performed entirely within Delaware.

      2.2 SEVERABILITY. In case any provision of this First Amendment is
determined to be invalid, illegal, or unenforceable, the validity, legality, and
enforceability of the remaining provisions will not in any way be affected or
impaired thereby.

      2.3 TITLES AND SUBTITLES. The titles of the sections of this First
Amendment are for convenience of reference only and are not to be considered in
construing this Agreement.

      2.4 COUNTERPARTS. This First Amendment may be executed in any number of
counterparts, each of which is an original, and all of which together
constitutes one instrument.

         *THE REMAINDER OF THIS PAGE HAS BEEN INTENTIONALLY LEFT BLANK*

                                       -2-
<PAGE>

      IN WITNESS WHEREOF, the parties hereto have executed this First Amendment
to the Investor Rights Agreement as of the date first set forth above.

                                             COMPANY:

                                             NxSTAGE MEDICAL, INC.

                                             By:  /s/ Jeffrey H. Burbank
                                                 ----------------------------
                                                 Name: Jeffrey H. Burbank
                                                 Title: President

                                             INVESTORS:

                                              /s/ Jeffrey H. Burbank
                                             --------------------------------
                                             Jeffrey H. Burbank

                                              /s/ David S. Utterberg
                                             --------------------------------
                                             David S. Utterberg

                                       -3-
<PAGE>
                                           ATLAS VENTURE FUND III, L.P.

                                           By: Atlas Venture Associates III, LLC
                                               Its General Partner

                                           By:  /s/ Jean-Francois Forinela
                                               ----------------------------
                                               Name: Jean-Francois Forinela
                                               Title: Member Manager

                                           ATLAS VENTURE ENTREPRENEURS'
                                           FUND III, L.P.

                                           By: Atlas Venture Associates III, LLC
                                               Its General Partner

                                           By:  /s/ Jean-Francois Forinela
                                               ----------------------------
                                               Name: Jean-Francois Forinela
                                               Title: Member Manager

                                      -4-
<PAGE>
                                           WPG ENTERPRISE FUND III, L.L.C.

                                           By: WPG VC FUND ADVISER, L.L.C.,
                                               Fund Investment Advisory Member

                                           By: /s/ Jeani Delagardelle
                                               ----------------------------
                                               Name: Jeani Delagardelle
                                               Title: Managing Member

                                           WEISS, PECK & GREER VENTURE
                                           ASSOCIATES IV, L.L.C.

                                           By: WPG VC Fund Adviser, L.L.C.,
                                               Fund Investment Advisory Member

                                           By: /s/ Jeani Delagardelle
                                               ----------------------------
                                               Name: Jeani Delagardelle
                                               Title: Managing Member

                                           WEISS, PECK & GREER VENTURE
                                           ASSOCIATES IV CAYMAN, L.P.

                                           By: WPG VC Fund Adviser, L.L.C.,
                                               Fund Investment Advisory Member

                                           By: /s/ Jeani Delagardelle
                                               ----------------------------
                                               Name: Jeani Delagardelle
                                               Title: Managing Member

                                      -5-
<PAGE>
                                        SPROUT CAPITAL VIII, L.P.

                                        By: DJL CAPITAL CORP.
                                            Its Managing General Partner

                                        By: /s/ Philippe O. Chambon
                                            ----------------------------
                                            Name: Philippe O. Chambon
                                            Title: Attorney In Fact

                                        SPROUT VENTURE CAPITAL, L.P.

                                        By: DJL CAPITAL CORP.
                                            Its Managing General Partner

                                        By: /s/ Philippe O. Chambon
                                            ----------------------------
                                            Name: Philippe O. Chambon
                                            Title: Attorney In Fact

                                        DLJ CAPITAL CORP.

                                        By: /s/ Philippe O. Chambon
                                            ----------------------------
                                            Name: Philippe O. Chambon
                                            Title: Attorney In Fact

                                        DLJ ESC II, L.P.

                                        By: DLJ LBO Plans Management Corporation
                                            Its General Partner

                                        By: /s/ Philippe O. Chambon
                                            ----------------------------
                                            Name: Philippe O. Chambon
                                            Title: Attorney In Fact

                                      -6-
<PAGE>

                              NxSTAGE MEDICAL, INC.

                             SECOND AMENDMENT TO THE
                            INVESTOR RIGHTS AGREEMENT

      THIS SECOND AMENDMENT TO THE INVESTOR RIGHTS AGREEMENT (this "Second
Amendment") is entered into as of the 24th day of May, 2001, by and among
NxSTAGE MEDICAL, INC., a Delaware corporation (the "Company"), and certain of
the purchasers of the Company's Series B Preferred Stock, par value $0.001 per
share (the "Series B Preferred Stock"), the Company's Series C Preferred Stock,
par value $0.001 per share (the "Series C Preferred Stock"), and all of the
purchasers of the Company's Series D Preferred Stock, par value $0.001 per share
(the "Series D Preferred Stock"), set forth on the signature pages hereto
(hereinafter, the "Investors" and each individually an "Investor").

                                    RECITALS

      WHEREAS, certain of the Investors purchased, in the aggregate, 1,875,000
shares of Series B Preferred Stock pursuant to the Series B Preferred Stock
Purchase Agreement dated as of June 30, 1999 (the "Series B Purchase
Agreement");

      WHEREAS, in connection with the acquisition of the Series B Preferred
Stock, the Company and certain of the Investors entered into an Investor Rights
Agreement dated as of June 30, 1999 (the "Investor Rights Agreement");

      WHEREAS, certain of the Investors purchased, in the aggregate, 1,151,632
shares of Series C Preferred Stock pursuant to the Series C Preferred Stock
Purchase Agreement dated as of January 24, 2001 ("the Series C Purchase
Agreement").

      WHEREAS, in connection with the acquisition of the Series C Preferred
Stock, the Company and certain of the Investors entered into a First Amendment
to the Investor Rights Agreement dated as of January 24, 2000 (the "First
Amendment").

      WHEREAS, the Company and certain of the Investors have entered into a
Series D Preferred Stock Purchase Agreement of even date herewith (the "Series D
Purchase Agreement"), pursuant to which each of the Investors will purchase that
number of shares of Series D Preferred Stock set forth opposite his or its name
on Exhibit A totaling an aggregate of 4,857,622 shares of Series D Preferred
Stock; and

      WHEREAS, it is a condition to the closing of the sale of the Series D
Preferred Stock, pursuant to the Series D Purchase Agreement, that the Company
grant to the Investors, with respect to the Series D Preferred Stock, those
registration rights, information rights and other rights as set forth in the
Investor Rights Agreement, as amended by the First Amendment;

      WHEREAS, the Investors executing this Second Amendment are the holders of
at least 66 2/3 % of the Registrable Securities, as defined in the Investor
Rights Agreement as amended.

<PAGE>

      NOW THEREFORE, in consideration of the mutual promises, representations,
warranties, covenants and conditions set forth herein and in the Series D
Purchase Agreement, the parties hereby agree as follows:

1.    SECOND AMENDMENT TO THE INVESTOR RIGHTS AGREEMENT. In accordance with
Section 5.5 of the Investors Agreement, as amended by the First Amendment, the
Investors and the Company hereby agree to amend the Investor Rights Agreement as
follows:

      a. The definition of "Shares" set forth in Section 1 of the Investor
Rights Agreement is amended to read in its entirety as follows:

      "SHARES" means the Company's Series B Preferred Stock issued pursuant to
the Series B Purchase Agreement, the Company's Series C Preferred Stock issued
pursuant to the Series C Purchase Agreement and the Company's Series D Preferred
Stock issued pursuant to the Series D Purchase Agreement.

      b. All references to "Shares" throughout the Investor Rights Agreement
shall be read to refer to the amended definition of "Shares" as set forth above.

      c. Section 5.5 a. of the Agreement is amended to read in its entirety as
follows:

      "Except as otherwise expressly provided, this Agreement may be amended or
      modified only upon the written consent of the Company and the holders of
      at least two-thirds (662/3%) of the Registrable Securities.
      Notwithstanding the foregoing, this Agreement may not be amended without
      the prior written consent of eighty-five percent (85%) of the Series D
      Preferred Stock if such amendment adversely applies to the holders of the
      Series D Preferred Stock in a different manner from the holders of any
      other series of Preferred Stock.

      d. Exhibit A to the Investor Rights Agreement is amended and restated as
set forth on the Exhibit A attached hereto.

      e. The remainder of the Investor Rights Agreement not amended hereby
remains unchanged and in full force and effect.

2.    MISCELLANEOUS

      2.1. GOVERNING LAW. This Second Amendment is governed by and construed
under the laws of the State of Delaware as applied to agreements among Delaware
residents entered into and to be performed entirely within Delaware.

      2.2. SEVERABILITY. In case any provision of this Second Amendment is
determined to be invalid, illegal, or unenforceable, the validity, legality, and
enforceability of the remaining provisions will not in any way be affected or
impaired thereby.

      2.3. TITLES AND SUBTITLES. The titles of the sections of this Second
Amendment are for convenience of reference only and are not to be considered in
construing this Agreement.

                                       -2-
<PAGE>

      2.4. COUNTERPARTS. This Second Amendment may be executed in any number of
counterparts, each of which is an original, and all of which together
constitutes one instrument.

         *THE REMAINDER OF THIS PAGE HAS BEEN INTENTIONALLY LEFT BLANK*

                                       -3-
<PAGE>

      IN WITNESS WHEREOF, the parties hereto have executed this First Amendment
to the Investor Rights Agreement as of the date first set forth above.

                                      COMPANY

                                      NxSTAGE MEDICAL, INC.
                                      439 SOUTH UNION STREET, 5TH FLOOR
                                      LAWRENCE, MA 01843

                                      By: /s/ Jeffrey H. Burbank
                                          ----------------------
                                          Name: Jeffrey H. Burbank
                                          Title: President

                                      INVESTORS:

                                      /s/ Jeffrey H. Burbank
                                      --------------------------
                                                Jeffrey H. Burbank

                                      /s/ David S. Utterberg
                                      --------------------------
                                                David S. Utterberg

                                      /s/ Paul Brown
                                      --------------------------
                                                Paul Brown

                                      -4-
<PAGE>

                                      WPG ENTERPRISE FUND III, L.L.C

                                      By: WPG VC Fund Adviser, L.L.C.,
                                          Fund Investment Advisory Member

                                      By: [illegible]
                                          ----------------------
                                          Name:
                                          Title:

                                      WEISS, PECK & GREER VENTURE
                                      ASSOCIATES IV, L.L.C.

                                      By: WPG VC Fund Adviser, L.L.C.,
                                          Fund Investment Advisory Member

                                      By: /s/ [illegible]
                                          ----------------------
                                          Name:
                                          Title:

                                      WEISS, PECK & GREER VENTURE
                                      ASSOCIATES IV CAYMAN, L.P.

                                      By: WPG VC Fund Adviser, L.L.C.,
                                          Fund Investment Advisory Partner

                                      By: /s/ [illegible]
                                          ----------------------
                                          Name:
                                          Title:

                                      -5-
<PAGE>

                                      ATLAS VENTURE FUND III, L.P.

                                      By: Atlas Venture Associates III, L.P.
                                          Its General Partner

                                      By: Atlas Venture Associates III, Inc.
                                          Its General Partner

                                      By: /s/ [illegible]
                                          ----------------------
                                          Name:
                                          Title: Vice President

                                      ATLAS VENTURE ENTREPRENEURS'
                                      FUND III, L.P.

                                      By: Atlas Venture Associates III, L.P.
                                          Its General Partner

                                      By: Atlas Venture Associates III, Inc.
                                          Its General Partner

                                      By: /s/ [illegible]
                                          ----------------------
                                          Name:
                                          Title: Vice President

                                      ATLAS VENTURE FUND V, L.P.

                                      By: Atlas Venture Associates V, L.P.
                                          Its General Partner

                                      By: Atlas Venture Associates V, Inc.
                                          Its General Partner

                                      By: /s/ [illegible]
                                          ----------------------
                                          Name:
                                          Title: Vice President

                                      -6-
<PAGE>

                                      ATLAS VENTURE PARALLEL FUND
                                      V-A C.V.

                                      By: Atlas Venture Associates V, L.P.
                                          Its General Partner

                                      By: Atlas Venture Associates V, Inc.
                                          Its General Partner

                                      By: /s/ [illegible]
                                          ----------------------
                                          Name:
                                          Title: Vice President

                                      ATLAS VENTURE PARALLEL FUND
                                      V-B C.V.

                                      By: Atlas Venture Associates V, L.P.
                                          Its General Partner

                                      By: Atlas Venture Associates V, Inc.
                                          Its General Partner

                                      By: /s/ [illegible]
                                          ----------------------
                                          Name:
                                          Title: Vice President

                                      ATLAS VENTURE ENTREPRENEURS'
                                      FUND V, L.P.

                                      By: Atlas Venture Associates V, L.P.
                                          Its General Partner

                                      By: Atlas Venture Associates V, Inc.
                                          Its General Partner

                                      By: /s/ [illegible]
                                          ----------------------
                                          Name:
                                          Title: Vice President

                                      -7-
<PAGE>

                                      SPROUT CAPITAL VIII, L.P.

                                      By: DLJ CAPITAL CORP.
                                          Its Managing General Partner

                                      By: /s/ Philippe O. Chambon
                                          -------------------------------
                                          Name: Philippe O. Chambon
                                          Title: Managing Director

                                      SPROUT VENTURE CAPITAL, L.P.

                                      By: DLJ CAPITAL CORP.
                                          Its Managing General Partner

                                      By: /s/ Philippe O. Chambon
                                          -------------------------------
                                          Name: Philippe O. Chambon
                                          Title: Attorney In Fact

                                      DLJ CAPITAL CORP.

                                      By: /s/ Philippe O. Chambon
                                          -------------------------------
                                          Name: Philippe O. Chambon
                                          Title: Attorney In Fact

                                      DLJ ESC II, L.P.

                                      By: DLJ LBO Plans Management
                                          Corporation
                                          Its General Partner

                                      By: /s/ Philippe O. Chambon
                                          -------------------------------
                                          Name: Philippe O. Chambon
                                          Title: Attorney In Fact

                                      SPROUT ENTREPRENEURS FUND, L.P.

                                      By: DLJ Capital Corp.
                                          Its General Partner

                                      By: /s/ Philippe O. Chambon
                                          -------------------------------
                                          Name: Philippe O. Chambon
                                          Title: Managing Director

                                      -8-
<PAGE>

                                      SPROUT CAPITAL IX, L.P.

                                      By: DLJ Capital Corp.
                                          Its Managing General Partner

                                      By: /s/ Philippe O. Chambon
                                          -------------------------------
                                          Name: Philippe O. Chambon
                                          Title: Managing Director

                                      -9-
<PAGE>

<TABLE>
<S>                                        <C>              <C>            <C>
WPG Enterprise Fund III, L.L.C             132,054           90,133           65,895
c/o Weiss, Peck & Greer, LLC
555 California Street, Suite 3130
San Francisco, California 94104

Weiss, Peck & Greer Venture                151,057           96,481           75,377
Associates IV, L.L.C.
c/o Weiss, Peck & Greer, LLC
555 California Street, Suite 3130
San Francisco, CA 94104

Weiss, Peck & Greer Venture                 19,003            6,348            9,482
Associates IV Cayman, L.P.
c/o Weiss, Peck & Greer, LLC
555 California Street, Suite 3130
San Francisco, CA 94104

David S. Utterberg                         201,410          128,642          753,769
c/o Medisystems Corporation
1201 Third Avenue, 39th Floor
Seattle, Washington 98101-3016

Atlas Venture Fund III, L.P.               164,268          104,920           72,536
222 Berkeley Street, Suite 1950
Boston, Massachusetts 02116

Atlas Venture Entrepreneurs'                 3,572            2,281            1,577
Fund III, L.P.
222 Berkeley Street, Suite 1950
Boston, Massachusetts 02116

Atlas Venture Fund V, L.P.                 164,268          104,920        1,133,073
222 Berkeley Street, Suite 1950
Boston, Massachusetts 02116

Atlas Venture Parallel                       3,572            2,281          140,746
Fund V-A C.V.
222 Berkeley Street, Suite 1950
Boston, Massachusetts 02116

Atlas Venture Parallel                         -0-              -0-          140,746
Fund V-B C.V.
222 Berkeley Street, Suite 1950
Boston, Massachusetts 02116
</TABLE>

                                      -10-
<PAGE>

<TABLE>
<S>                                      <C>              <C>              <C>
Atlas Venture Entrepreneurs'                   -0-              -0-           18,860
Fund V L.P.
222 Berkeley Street, Suite 1950
Boston, Massachusetts 02116

Paul Brown                                     -0-              -0-           16,750
115 Arbor Court
Woodside, CA 94062

TOTAL                                    1,875,000        1,151,632        4,857,622
</TABLE>

                                      -11-
<PAGE>

                              NxSTAGE MEDICAL, INC.

                             THIRD AMENDMENT TO THE

                            INVESTOR RIGHTS AGREEMENT

      THIS THIRD AMENDMENT TO THE INVESTOR RIGHTS AGREEMENT (this "Third
Amendment") is entered into as of the 15th day of April, 2003, by and among
NxSTAGE MEDICAL, INC., a Delaware corporation (the "Company"), and certain of
the purchasers of the Company's Series B Preferred Stock, par value $0.001 per
share (the "Series B Preferred Stock"), Series C Preferred Stock, par value
$0.001 per share (the "Series C Preferred Stock"), Series D Preferred Stock, par
value $0.001 per share (the "Series D Preferred Stock") and all of the
purchasers of the Company's Series E Preferred Stock, par value $0.001 per share
(the "Series E Preferred Stock"), set forth on the signature pages hereto
(hereinafter, the "Investors" and each individually an "Investor").

                                    RECITALS

      WHEREAS, certain of the Investors purchased, in the aggregate, 1,875,000
shares of Series B Preferred Stock pursuant to the Series B Preferred Stock
Purchase Agreement dated as of June 30, 1999 (the "Series B Purchase
Agreement");

      WHEREAS, in connection with the acquisition of the Series B Preferred
Stock, the Company and certain of the Investors entered into an Investor Rights
Agreement dated as of June 30, 1999 (the "Investor Rights Agreement");

      WHEREAS, certain of the Investors purchased, in the aggregate, 1,151,632
shares of Series C Preferred Stock pursuant to the Series C Preferred Stock
Purchase Agreement dated as of January 24, 2001 ("the Series C Purchase
Agreement");

      WHEREAS, in connection with the acquisition of the Series C Preferred
Stock, the Company and certain of the Investors entered into a First Amendment
to the Investor Rights Agreement dated as of January 24, 2000 (the "First
Amendment");

      WHEREAS, certain of the Investors purchased, in the aggregate, 4,857,622
shares of Series D Preferred Stock pursuant to the Series D Preferred Stock
Purchase Agreement dated as of May 24, 2001 (the "Series D Purchase Agreement");

      WHEREAS, in connection with the acquisition of the Series D Preferred
Stock, the Company and certain Investors entered into a Second Amendment to the
Investor Rights Agreement dated as of May 24, 2001 (the "Second Amendment");

      WHEREAS, the Company and certain of the Investors have entered into a
Series E Preferred Stock Purchase Agreement of even date herewith (the "Series E
Purchase Agreement"), pursuant to which each of the Investors will purchase that
number of shares of Series E Preferred Stock set forth opposite his or its name
on Exhibit A totaling an aggregate of 669,908 shares of Series E Preferred
Stock; and

<PAGE>

      WHEREAS, it is a condition to the closing of the sale of the Series E
Preferred Stock, pursuant to the Series E Purchase Agreement, that the Company
grant to the Investors, with respect to the Series E Preferred Stock, those
registration rights, information rights and other rights as set forth in the
Investor Rights Agreement, as amended by the First and Second Amendments;

      WHEREAS, the Investors executing this Third Amendment are the holders of
at least 66 2/3% of the Registrable Securities, as defined in the Investor
Rights Agreement as amended by the First and Second Amendments.

      NOW THEREFORE, in consideration of the mutual promises, representations,
warranties, covenants and conditions set forth herein and in the Series E
Purchase Agreement, the parties hereby agree as follows:

1.    THIRD AMENDMENT TO THE INVESTOR RIGHTS AGREEMENT. In accordance with
Section 5.5 of the Investors Agreement, as amended by the First and Second
Amendments, the Investors and the Company hereby agree to amend the Investor
Rights Agreement as follows:

      a.    The definition of "Shares" set forth in Section 1 of the Investor
Rights Agreement is amended to read in its entirety as follows:

            "SHARES" means the Company's Series B Preferred Stock issued
pursuant to the Series B Purchase Agreement, the Company's Series C Preferred
Stock issued pursuant to the Series C Purchase Agreement, the Company's Series D
Preferred Stock issued pursuant to the Series D Purchase Agreement and the
Company's Series E Preferred Stock issued pursuant to the Series E Purchase
Agreement.

      b.    All references to "Shares" throughout the Investor Rights Agreement
shall be read to refer to the amended definition of "Shares" as set forth above.

      c.    Sections 4.1, 4.2, and 4.3 of the Agreement are amended to read in
their entirety as follows:

            "4.1 SUBSEQUENT OFFERINGS. Each Investor shall have a right of first
            refusal to purchase its Adjusted Pro Rata Share (as defined below)
            of all Equity Securities that the Company may, form time to time,
            propose to sell and issue after the date of this Agreement, other
            than the Equity Securities excluded by Section 4.6 hereof. For
            purposes hereof, pro rata share shall mean the quotient of the
            holder's total number of issued and outstanding shares in the
            Corporation (Preferred and Common) on a fully converted basis,
            divided by the total number of issued and outstanding shares in the
            Corporation (Preferred and Common) on a fully converted basis. Each
            Investor's Adjusted Pro Rata Share, shall be equal to its pro rata
            share, multiplied by a fraction, the numerator of which is the
            actual number of shares of Series E Preferred Stock held by such
            Investor, the denominator of which is the shares of Series E

                                      -2-
<PAGE>

            Preferred Stock associated with the Investor's name in the following
            table (in no event shall the resultant fraction exceed 100%):

<TABLE>
<S>                     <C>
    Sprout Group        987,552.0
Lightspeed (WPG)        201,338.0
 David Utterberg        270,592.0
   Atlas Venture        409,062.0
    Adams Street        157,345.0
      Paul Brown          3,496.0
</TABLE>

            4.2 EXERCISE OF RIGHTS. If the Company proposes to issue any Equity
            Securities, it all give each Investor written notice of its
            intention, describing the Equity Securities, the price and the terms
            and conditions upon which the Company proposes to issue the same.
            Each Investor shall have fifteen (15) days from the giving of such
            notice to agree to purchase his, her or its Adjusted Pro Rata share
            of the Equity Securities for the price and upon the terms and
            conditions specified in the notice by giving written notice to the
            Company and stating therein the quantity of Equity Securities to be
            purchased. Notwithstanding the foregoing, the Company shall not be
            required to offer or sell such Equity Securities to any Investor who
            would cause the Company to be in violation of applicable federal
            securities laws by virtue of such offer or sale.

            4.3 ISSUANCE OF EQUITY SECURITIES TO OTHER PERSONS. If not all of
            the Investors elect to purchase their Adjusted Pro Rata share of the
            Equity Securities, then the Company shall promptly notify in writing
            the Investors who do so elect and shall offer them the right to
            acquire their Adjusted Pro Rata shares of such unsubscribed shares.
            Such Investors shall have five (5) days after receipt of such notice
            to notify the Company of his, her or its election to purchase all or
            a portion thereof o f the unsubscribed shares. If the Investors so
            notified fail to exercise in full the rights of first refusal, the
            Company shall have ninety (90) days thereafter to sell the Equity
            Securities in respect of which the Investor's rights were not
            exercised, at a price and upon general terms and conditions
            materially no more favorable to the purchasers thereof than
            specified in the Company's notice to the Investors pursuant to
            Section 4.2 hereof. If the Company has not sold such Equity
            Securities within ninety (90) days of the notice provided pursuant
            to Section 4.2, the Company shall not thereafter issue or sell any
            Equity Securities. without first offering such securities to the
            Investors in the manner provided above."

                                      -3-
<PAGE>

            4.7 INELIGIBLE INVESTOR. Mr. Burbank shall not have rights of first
            refusal under this Article 4. Solely for purposes of this Article 4,
            Mr. Burbank shall not be included in the definition of Investor.

      d. Exhibit A to the Investor Rights Agreement is amended and restated as
set forth on the Exhibit A attached hereto.

      e. The remainder of the Investor Rights Agreement not amended hereby
remains unchanged and in full force and effect.

2.    MISCELLANEOUS

      2.1. GOVERNING LAW. This Third Amendment is governed by and construed
under the laws of the State of Delaware as applied to agreements among Delaware
residents entered into and to be performed entirely within Delaware.

      2.2. SEVERABILITY. In case any provision of this Third Amendment is
determined to be invalid, illegal, or unenforceable, the validity, legality, and
enforceability of the remaining provisions will not in any way be affected or
impaired thereby.

      2.3. TITLES AND SUBTITLES. The titles of the sections of this Third
Amendment are for convenience of reference only and are not to be considered in
construing this Agreement.

      2.4. COUNTERPARTS. This Third Amendment may be executed in any number of
counterparts, each of which is an original, and all of which together
constitutes one instrument.

         *THE REMAINDER OF THIS PAGE HAS BEEN INTENTIONALLY LEFT BLANK*

                                      -4-
<PAGE>

      IN WITNESS WHEREOF, the parties hereto have executed this Third Amendment
to the Investor Rights Agreement as of the date first set forth above.

                                        COMPANY:

                                        NxSTAGE MEDICAL, INC.
                                        439 SOUTH UNION STREET, 5TH FLOOR
                                        LAWRENCE, MA 01843

                                        By: /s/ Jeffrey H. Burbank
                                            -------------------------------
                                            Name: Jeffrey H.  Burbank
                                            Title: President

                                        INVESTORS:

                                        By: /s/ Jeffrey H. Burbank
                                            -------------------------------
                                                     Jeffrey H.  Burbank

                                            /s/ David S. Utterberg
                                            -------------------------------
                                                     David S. Utterberg

                                            /s/ Paul Brown
                                            -------------------------------
                                                     Paul Brown

                                       -5-
<PAGE>

                                        BVCF IV, L.P.

                                        By: J.W. Puth Associates, LLC
                                            Its General Partner

                                        By: Brinson Venture Management, LLC
                                            Its Attorney-In-Fact

                                        By: Adams Street Partners, LLC
                                            As its Administrative Member

                                        By: /s/ Craig S. Taylor
                                            --------------------------
                                            Name: Craig S. Taylor
                                            Title: Partner

                                       -6-
<PAGE>

                                        ATLAS VENTURE FUND III, L.P.

                                        By: Atlas Venture Associates III, L.P.
                                            Its General Partner

                                        By: Atlas Venture Associates III, Inc.
                                            Its General Partner

                                        By: /s/ [illegible]
                                            --------------------------
                                            Name:
                                            Title: Vice President

                                        ATLAS VENTURE ENTREPRENEURS'
                                        FUND III, L.P.

                                        By: Atlas Venture Associates III, L.P.
                                            Its General Partner

                                        By: Atlas Venture Associates III, Inc.
                                            Its General Partner

                                        By: /s/ [illegible]
                                            --------------------------
                                            Name:
                                            Title: Vice President

                                        ATLAS VENTURE FUND V, L.P.

                                        By: Atlas Venture Associates V, L.P.
                                            Its General Partner

                                        By: Atlas Venture Associates V, Inc.
                                            Its General Partner

                                        By: /s/ [illegible]
                                            --------------------------
                                            Name:
                                            Title: Vice President

                                      -7-
<PAGE>

                                        ATLAS VENTURE PARALLEL FUND V-A C.V.

                                        By: Atlas Venture Associates V, L.P.
                                            Its General Partner

                                        By: Atlas Venture Associates V, Inc.
                                            Its General Partner

                                        By: /s/ [illegible]
                                            --------------------------
                                            Name:
                                            Title: Vice President

                                        ATLAS VENTURE PARALLEL FUND V-B C.V.

                                        By: Atlas Venture Associates V, L.P.
                                            Its General Partner

                                        By: Atlas Venture Associates V, Inc.
                                            Its General Partner

                                        By: /s/ [illegible]
                                            --------------------------
                                            Name:
                                            Title: Vice President

                                        ATLAS VENTURE ENTREPRENEURS'
                                        FUND V, L.P.

                                        By: Atlas Venture Associates V, L.P.
                                            Its General Partner

                                        By: Atlas Venture Associates V, Inc.
                                            Its General Partner

                                        By: /s/ [illegible]
                                            --------------------------
                                            Name:
                                            Title: Vice President

                                      -8-
<PAGE>

                                        SPROUT CAPITAL VIII, L.P.

                                        By: DLJ Capital Corporation
                                            Its Managing General Partner

                                        By: /s/ Jeani Delagardelle
                                            --------------------------
                                            By: Jeani Delagardelle
                                            Its: Managing Director

                                        SPROUT VENTURE CAPITAL, L.P.

                                        By: DLJ Capital Corporation
                                            Its Managing General Partner

                                        By: /s/ Jeani Delagardelle
                                            --------------------------
                                            By: Jeani Delagardelle
                                            Its: Managing Director

                                        DLJ CAPITAL CORP.

                                        By: /s/ Jeani Delagardelle
                                            --------------------------
                                            By: Jeanie Delagardelle
                                            Its: Managing Director

                                        DLJ CAPITAL CORP.

                                        By: /s/ Jeani Delagardelle
                                            --------------------------
                                            By: Jeanie Delagardelle
                                            Its: Managing Director

                                        DLJ ESC II, L.P.

                                        By: DLJ LBO Plans Management
                                              Corporation
                                            Its General Partner

                                        By: /s/ Jeani Delagardelle
                                            --------------------------
                                            By: Jeani Delagardelle
                                            Its: Attorney In Fact

                                      -9-
<PAGE>

                                        SPROUT ENTREPRENEURS FUND, L.P.

                                        By: DLJ Capital Corp.
                                            Its General Partner

                                        By: /s/ Jeani Delagardelle
                                            --------------------------
                                            By: Jeani Delagardelle
                                            Its: Managing Director

                                        SPROUT CAPITAL IX, L.P.

                                        By: DLJ Capital Corp.
                                            Its Managing General Partner

                                        By: /s/ Jeani Delagardelle
                                            --------------------------
                                            By: Jeani Delagardelle
                                            Its: Managing Director

                                        SPROUT IX PLAN INVESTORS, L.P.

                                        By: DLJ LBO Plans Management Corporation
                                            Its General Partner

                                        By: /s/ Jeani Delagardelle
                                            --------------------------
                                            By: Jeani Delagardelle
                                            Its: Attorney In Fact

                                        THE SPROUT CEO FUND, L.P.

                                        By: DLJ Capital Corporation
                                            Its General Partner

                                        By: /s/ Jeani Delagardelle
                                            --------------------------
                                            By: Jeani Delagardelle
                                            Its: Managing Director

                                      -10-
<PAGE>

                                        SPROUT CAPITAL VII, L.P.

                                        By: DLJ Capital Corporation
                                            Its Managing General Partner

                                        By: /s/ Jeani Delagardelle
                                            --------------------------
                                            By: Jeani Delagardelle
                                            Its: Managing Director

                                      -11-
<PAGE>

                                    EXHIBIT A

                                LIST OF INVESTORS

<TABLE>
<CAPTION>
                                       No. OF SHARES    No. OF SHARES    No. OF SHARES    No. OF SHARES
                                            OF               OF               OF               OF
                                         SERIES B         SERIES C         SERIES D         SERIES E
                                      PREFERRED STOCK  PREFERRED STOCK  PREFERRED STOCK  PREFERRED STOCK
<S>                                   <C>              <C>              <C>              <C>
Jeffrey H.  Burbank                         74,906              -0-              -0-              -0-
c/o NxSTAGE MEDICAL, INC.
3 Highwood Drive
Tewksbury, MA 01876

BVCF IV, L.P.                                  -0-              -0-          753,769          157,345
209 South LaSalle Street
Chicago, IL 60604-1295

Sprout Capital VIII, L.P.                  981,202          628,353              -0-          354,895
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

Sprout Venture Capital, L.P.                58,872           37,732              -0-           21,300
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

DLJ Capital Corp.                            3,273            2,096           18,910           36,094
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

DLJ ESC II, L.P.                            85,383           54,646              -0-           30,756
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

Sprout Entrepreneurs Fund, L.P.                -0-              -0-            6,148            1,288
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

Sprout Capital IX, L.P.                        -0-              -0-        1,559,935          326,824
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

The Sprout CEO Fund, L.P.                      -0-              -0-              -0-            2,273
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

Sprout Capital VII, L.P.                       -0-              -0-              -0-          195,256
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025
</TABLE>

                                      -12-
<PAGE>

<TABLE>
<S>                                      <C>              <C>              <C>              <C>
Sprout IX Plan Investors                       -0-              -0-           90,049           18,866
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

WPG Enterprise Fund III, L.L.C.            132,054           84,344           65,895           10,983
c/o Weiss, Peck & Greer, LLC
555 California Street, Suite 3130
San Francisco, California 94104

Weiss, Peck & Greer Venture                151,057           96,481           75,377           12,563
  Associates IV, L.L.C.
c/o Weiss, Peck & Greer, LLC
555 California Street, Suite 3130
San Francisco, California 94104

Weiss, Peck & Greer Venture                 19,003           12,137            9,482            1,580
  Associates IV Cayman, L.P.
c/o Weiss, Peck & Greer, LLC
555 California Street, Suite 3130
San Francisco, California 94104

David S. Utterberg                         201,410          128,642          753,642          753,769
c/o Medisystems Corporation
1201 Third Avenue, 39th Floor
Seattle, Washington 98101-3016

Atlas Venture Fund III, L.P.               164,268          104,920           72,536          192,783
222 Berkeley Street, Suite 1950
Boston, Massachusetts 02116

Atlas Venture Entrepreneurs'                 3,572            2,281            1,577            4,192
  Fund, L.P.
222 Berkeley Street, Suite 1950
Boston, Massachusetts 02116

Atlas Venture Fund V, L.P.                     -0-              -0-        1,133,073          424,146
222 Berkeley Street, Suite 1950
Boston, Massachusetts 02116

Atlas Venture Parallel Fund V-A                -0-              -0-          140,746           52,686
  C.V.
222 Berkeley Street, Suite 1950
Boston, Massachusetts 02116

Atlas Venture Parallel Fund V-B                -0-              -0-          140,746           52,686
  C.V.
222 Berkeley Street, Suite 1950
Boston, Massachusetts 02116

Atlas Venture Entrepreneurs'                   -0-              -0-           18,860            7,060
  Fund V L.P.
222 Berkeley Street, Suite 1950
Boston, Massachusetts 02116

Paul Brown                                     -0-              -0-           16,750           12,563
115 Arbor Court
Woodside, CA 94062

TOTAL                                    1,876,000        1,151,632        4,857,622        2,669,908
</TABLE>

                                      -13-
<PAGE>

                              NxSTAGE MEDICAL, INC.

                             FOURTH AMENDMENT TO THE

                            INVESTOR RIGHTS AGREEMENT

         THIS FOURTH AMENDMENT TO THE INVESTOR RIGHTS AGREEMENT (this "Fourth
Amendment") is entered into as of the 18th day of August, 2004, by and among
NxSTAGE MEDICAL, INC., a Delaware corporation (the "Company"), and certain of
the purchasers of the Company's Series B Preferred Stock, par value $0.001 per
share (the "Series B Preferred Stock"), Series C Preferred Stock, par value
$0.001 per share (the "Series C Preferred Stock"), Series D Preferred Stock, par
value $0.001 per share (the "Series D Preferred Stock"), Series E Preferred
Stock, par value $0.001 per share (the "Series E Preferred Stock") and all of
the purchasers of the Company's Series F Preferred Stock, par value $0.001 per
share (the "Series F Preferred Stock"), set forth on the signature pages hereto
(hereinafter, the "Investors") and each individually an "Investor").

                                    RECITALS

         WHEREAS, certain of the Investors purchased, in a the aggregate,
1,875,000 shares of Series B Preferred Stock pursuant to the Series B Preferred
Stock Purchase Agreement dated as of June 30, 1999 (the "Series B Purchase
Agreement");

         WHEREAS, in connection with the acquisition of the Series B Preferred
Stock, the Company and certain of the Investors entered into an Investor Rights
Agreement dated as of June 30, 1999 (the "Investor Rights Agreement");

         WHEREAS, certain of the Investors purchased, in the aggregate,
1,151,632 shares of Series C Preferred Stock pursuant to the Series C Preferred
Stock Purchase Agreement dated as of January 24, 2001 ("the Series C Purchase
Agreement");

         WHEREAS, in connection with the acquisition of the Series C Preferred
Stock, the Company and certain of the Investors entered into a First Amendment
to the Investor Rights Agreement dated as of January 24, 2000 (the "First
Amendment");

         WHEREAS, certain of the Investors purchased, in the aggregate,
4,857,622 shares of Series D Preferred Stock pursuant to the Series D Preferred
Stock Purchase Agreement dated as of May 24, 2001 (the "Series D Purchase
Agreement");

         WHEREAS, in connection with the acquisition of the Series D Preferred
Stock, the Company and certain Investors entered into a Second Amendment to the
Investor Rights Agreement dated as of May 24, 2001 (the "Second Amendment");

         WHEREAS, certain of the Investors purchased, in the aggregate,
2,669,908 shares of Series E Preferred Stock pursuant to the Series E Preferred
Stock Purchase Agreement dated as of April 15, 2003 (the "Series E Purchase
Agreement");

<PAGE>

         WHEREAS, in connection with the acquisition of the Series E Preferred
Stock, the Company and certain Investors entered into a Third Amendment to the
Investor Rights Agreement dated as of April 15, 2003 (the "Third Amendment");

         WHEREAS, the Company and certain of the Investors have entered into a
Series F Preferred Stock Purchase Agreement of even date herewith (the "Series F
Purchase Agreement"), pursuant to which each of the Investors will purchase that
number of shares of Series F Preferred Stock set forth opposite his or its name
on Exhibit A totaling an aggregate of 2,747,253 shares of Series F Preferred
Stock; and

         WHEREAS, it is a condition to the closing of the sale of the Series F
Preferred Stock, pursuant to the Series F Purchase Agreement, that the Company
grant to the Investors, with respect to the Series F Preferred Stock, those
registration rights, information rights and other rights as set forth in the
Investor Rights Agreement, as amended by the First, Second and Third Amendments;

         WHEREAS, the Investors executing this Fourth Amendment are the holders
of at least 66 2/3 % of the Registrable Securities, as defined in the Investor
Rights Agreement as amended by the First and Second Amendments.

         NOW THEREFORE, in consideration of the mutual promises,
representations, warranties, covenants and conditions set forth herein and in
the Series F Purchase Agreement, the parties hereby agree as follows:

1.      FOURTH AMENDMENT TO THE INVESTOR RIGHTS AGREEMENT. In accordance with
Section 5.5 of the Investors Agreement, as amended by the First, Second and
Third Amendments, the Investors and the Company hereby agree to amend the
Investor Rights Agreement as follows:

         a. The definition of "Shares" as set forth in Section 1 of the Investor
Rights Agreement is amended to read in its entirety as follows:

            "SHARES" means the Company's Series B Preferred Stock issued
pursuant to the Series B Purchase Agreement, the Company's Series C Preferred
Stock issued pursuant to the Series C Purchase Agreement, the Company's Series D
Preferred Stock issued pursuant to the Series D Purchase Agreement, the
Company's Series E Preferred Stock issued pursuant to the Series E Purchase
Agreement and the Company's Series F Preferred Stock issued pursuant to the
Series F Purchase Agreement.

         b. All references to "Shares" throughout the Investor Rights Agreement
shall be read to refer to the amended definition of "Shares" as set forth above.

         c. Section 2.4(b)(iv) of the Agreement shall be deleted in its
entirety.

         d. Section 4.1 of the Agreement is amended to read in its entirety as
follows:

         4.1 SUBSEQUENT OFFERINGS. Each Investor shall have a right of first
refusal to purchase its Adjusted Pro Rata Share (as defined below) of all Equity
Securities that the

                                      -2-
<PAGE>

Company may, from time to time, propose to sell and issue after the date of this
Agreement, other than the Equity Securities excluded by Section 4.6 hereof. For
purposes hereof, pro rata shares shall mean the quotient of the holder's total
number of issued and outstanding shares in the Corporation (Preferred and
Common) on a fully converted basis, divided by the total number of issued and
outstanding shares in the Corporation (Preferred and Common) on a fully
converted basis. Each Investor's Adjusted Pro Rata Share shall be equal to its
pro rata share, multiplied by a fraction, the numerator of which is the actual
number of shares of Series F Preferred Stock held by such Investor, and the
denominator of which is the shares of Series F Preferred Stock associated with
the Investor's name in the following table (in no event shall the resultant
fraction exceed 100%):

<TABLE>
<S>                                       <C>
Sprout Group                              643,134
WPG                                       111,301
David Utterberg                           230,562
Atlas Ventures                            302,892
Adams Street                              102,470
Paul Brown                                  3,297
Marubeni                                  412,088
CSFB                                      274,725
Wasatch                                   206,044
</TABLE>

         e. Exhibit A to the Investor Rights Agreement is amended and restated
as set forth on the Exhibit A attached hereto.

         e. The remainder of the Investor Rights Agreement not amended hereby
remains unchanged and in full force and effect.

2.       MISCELLANEOUS

         2.1 GOVERNING LAW. This Fourth Amendment is governed by and construed
under the laws of the State of Delaware as applied to agreements among Delaware
residents entered into and to be performed entirely within Delaware.

         2.2 SEVERABILITY. In case any provision of this Fourth Amendment is
determined to be invalid, illegal, or unenforceable, the validity, legality, and
enforceability of the remaining provisions will not in any way be affected or
impaired thereby.

         2.3 TITLES AND SUBTITLES. The titles of the sections of this Fourth
Amendment are for convenience of reference only and are not to be considered in
construing this Agreement.

         2.4 COUNTERPARTS. This Fourth Amendment may be executed in any number
of counterparts, each of which is an original, and all of which together
constitutes one instrument.

         *THE REMAINDER OF THIS PAGE HAS BEEN INTENTIONALLY LEFT BLANK*

                                      -3-
<PAGE>

         IN WITNESS WHEREOF, the parties hereto have executed this Fourth
Amendment to the Investor Rights Agreement as of the date first set forth above.

                                        COMPANY:

                                        NxSTAGE MEDICAL, INC.
                                        439 SOUTH UNION STREET, 5TH FLOOR
                                        LAWRENCE, MA  01843

                                        By:/s/ Jeffrey H. Burbank
                                           -------------------------------------
                                               Name: Jeffrey H. Burbank
                                               Title: President

                                        INVESTORS:

                                        /s/ Jeffrey H. Burbank
                                        ----------------------------------------
                                            Jeffrey H. Burbank

                                        /s/ David S. Utterberg
                                        ----------------------------------------
                                            David S. Utterberg

                                        /s/ Paul Brown
                                        ----------------------------------------
                                            Paul Brown

                                      -4-
<PAGE>

                                        MARUBENI CORPORATION

                                        By:/s/ Tetsuji Banno
                                           -------------------------------------
                                               Name: Tetsuji Banno
                                               Title: General Manager,
                                               Business Incubation Dept.

                                        MARUBENI AMERICA CORPORATION

                                        By:/s/ Shigemasa Sonobe
                                           -------------------------------------
                                               Name: Shigemasa Sonobe
                                               Title: General Manager,
                                               Investment Business Unit

                                      -5-
<PAGE>

                                        BVCF IV, L.P.

                                        By: Adams Street Partners, LLC,
                                            its General Partner

                                        By:/s/ Craig S. Taylor
                                           -------------------------------------
                                               Craig S. Taylor
                                               Partner

                                      -6-
<PAGE>

                                        WPG ENTERPRISE FUND III, L.L.C.

                                        By: WPG VC Fund Adviser, L.L.C.,
                                            Fund Investment Advisory Member

                                        By:/s/ Dave Markland
                                           -------------------------------------
                                               Name:  Dave Markland
                                               Title: CFO & Administrative
                                                      Partner

                                        WEISS, PECK & GREER VENTURE
                                        ASSOCIATES IV, L.L.C.

                                        By:    WPG VC Fund Adviser, L.L.C.,
                                               Fund Investment Advisory Member

                                        By:/s/ Dave Markland
                                           -------------------------------------
                                               Name:  Dave Markland
                                               Title: CFO & Administrative
                                                      Partner

                                        WEISS, PECK & GREER VENTURE
                                        ASSOCIATES IV CAYMAN, L.P.

                                        By:    WPG VC Fund Adviser, L.L.C.,
                                               General Partner

                                        By:/s/ Dave Markland
                                           -------------------------------------
                                               Name:  Dave Markland
                                               Title: CFO & Administrative
                                                      Partner

                                      -7-
<PAGE>

                                       ATLAS VENTURE FUND V, L.P.
                                       ATLAS VENTURE PARALLEL FUND V-A, C.V.
                                       ATLAS VENTURE PARALLEL FUND V-B, C.V.
                                       ATLAS VENTURE ENTREPRENEURS' FUND V, L.P.

                                       By:    Atlas Venture Associates V, L.P.
                                              their General Partner

                                       By:    Atlas Venture Associates V, Inc.
                                              Its General Partner

                                       By:/s/ [illegible]
                                          -------------------------------------
                                              Name:
                                              Title:   Vice President

                                       ATLAS VENTURE FUND III, L.P.
                                       ATLAS VENTURE ENTREPRENEURS'
                                       FUND III, L.P.

                                       By:   Atlas Venture Associates III, L.P.
                                             their General Partner

                                       By:   Atlas Venture Associates III, Inc.
                                             Its General Partner

                                       By:/s/ [illegible]
                                          -------------------------------------
                                              Name:
                                              Title:   Vice President

                                      -8-
<PAGE>

                                        SPROUT CAPITAL VIII, L.P.

                                        By: DLJ Capital Corporation
                                            Its Managing General Partner

                                        By:/s/ Philippe O. Chambon
                                           -------------------------------------
                                               By:  Philippe Chambon
                                               Its:  Managing Director

                                        SPROUT VENTURE CAPITAL, L.P.

                                        By: DLJ Capital Corporation
                                            Its Managing General Partner

                                        By:/s/ Philippe O. Chambon
                                           -------------------------------------
                                               By:  Philippe Chambon
                                               Its: Managing Director

                                        DLJ CAPITAL CORP.

                                        By:/s/ Philippe O. Chambon
                                           -------------------------------------
                                               By:  Philippe Chambon
                                               Its: Managing Director

                                        DLJ CAPITAL CORP.

                                        By:/s/ Philippe O. Chambon
                                           -------------------------------------
                                               By:  Philippe Chambon
                                               Its: Managing Director

                                        DLJ ESC II, L.P.

                                        By:    DLJ LBO Plans Management
                                               Corporation
                                               Its General Partner

                                        By:/s/ Philippe O. Chambon
                                           -------------------------------------
                                               By:  Philippe Chambon
                                               Its: Attorney In Fact

                                      -9-
<PAGE>

                                        SPROUT ENTREPRENEURS FUND, L.P.

                                        By:    DLJ Capital Corp.
                                               Its General Partner

                                        By:/s/ Philippe O. Chambon
                                           -------------------------------------
                                               By:  Philippe Chambon
                                               Its: Managing Director

                                        SPROUT CAPITAL IX, L.P.

                                        By:    DLJ Capital Corp.
                                               Its Managing General Partner

                                        By:/s/ Philippe O. Chambon
                                           -------------------------------------
                                               By:  Philippe Chambon
                                               Its: Managing Director

                                        SPROUT IX PLAN INVESTORS, L.P.

                                        By:    DLJ LBO Plans Management
                                               Corporation
                                               Its General Partner

                                        By:/s/ Philippe O. Chambon
                                           -------------------------------------
                                               By:  Philippe Chambon
                                               Its: Attorney In Fact

                                        SPROUT PLAN INVESTORS, L.P.

                                        By:    DLJ LBO Plans Management
                                               Corporation
                                               Its General Partner

                                        By:/s/ Philippe O. Chambon
                                           -------------------------------------
                                               By:  Philippe Chambon
                                               Its: Attorney In Fact

                                      -10-
<PAGE>

                                        THE SPROUT CEO FUND, L.P.

                                        By:    DLJ Capital Corporation
                                               Its General Partner

                                        By:/s/ Philippe O. Chambon
                                           -------------------------------------
                                               By:  Philippe Chambon
                                               Its: Managing Director

                                        SPROUT CAPITAL VII, L.P.

                                        By:    DLJ Capital Corporation
                                               Its Managing General Partner

                                        By:/s/ Philippe O. Chambon
                                           -------------------------------------
                                               By:  Philippe Chambon
                                               Its:  Managing Director

                                      -11-
<PAGE>

                                        WASATCH FUNDS, INC. FOR WASATCH ULTRA
                                        GROWTH FUND
                                        By:/s/ Venice F. Edwards
                                           -------------------------------------
                                               Name:  Venice F. Edwards
                                               Title: Secretary

                                      -12-
<PAGE>

                                        CSFB FUND CO-INVESTMENT PROGRAM, L.P.

                                        By:    DLJ Fund Partners, L.P.
                                               Its General Partner

                                        By:    DLJMB Fund, Inc.
                                               Its General Partner

                                        By:/s/ Edward Nadel
                                           -------------------------------------
                                           By:  Edward Nadel
                                           Its: Vice President

                                      -13-
<PAGE>

                                    EXHIBIT A

                                LIST OF INVESTORS

<TABLE>
<CAPTION>
                                     NO. OF SHARES    NO. OF SHARES   NO. OF SHARES    NO. OF SHARES   NO. OF SHARES
                                           OF              OF               OF              OF               OF
                                        SERIES B        SERIES C         SERIES D        SERIES E         SERIES F
                                       PREFERRED        PREFERRED       PREFERRED        PREFERRED       PREFERRED
         NAME AND ADDRESS                STOCK            STOCK           STOCK            STOCK           STOCK
---------------------------------    -------------    -------------   -------------    -------------   -------------
<S>                                  <C>              <C>             <C>              <C>             <C>
Jeffrey H. Burbank                          74,906              -0-             -0-              -0-             -0-
c/o NxSTAGE MEDICAL, INC.
3 Highwood Drive
Tewksbury, MA  01876

BVCF IV, L.P.                                  -0-              -0-         753,769          157,345          68,681
209 South LaSalle Street
Chicago, IL  60604-1295

Sprout Capital VIII, L.P.                  981,202          628,353             -0-          354,895         447,582
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

Sprout Venture Capital, L.P.                58,872           37,732             -0-           21,300          26,863
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

DLJ Capital Corp.                            3,273            2,096          18,910           36,094           6,489
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

DLJ ESC II, L.P.                            85,383           54,646             -0-           30,756             -0-
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

Sprout Entrepreneurs Fund, L.P.                -0-              -0-           6,148            1,288           1,624
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

Sprout Capital IX, L.P.                        -0-              -0-       1,559,935          326,824         412,180
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

The Sprout CEO Fund, L.P.                      -0-              -0-             -0-            2,273             -0-
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025
</TABLE>

                                      -14-
<PAGE>

<TABLE>
<S>                                        <C>              <C>           <C>                <C>             <C>
Sprout Capital VII, L.P.                       -0-              -0-             -0-          195,256             -0-
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

Sprout IX Plan Investors                       -0-              -0-          90,049           18,866          23,793
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

Sprout Plan Investors, L.P.                    -0-              -0-             -0-              -0-          38,788
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

WPG Enterprise Fund III, L.L.C.            132,054           84,344          65,895           10,983           9,006
c/o Weiss, Peck & Greer, LLC
555 California Street, Suite 3130
San Francisco, California 94104

Weiss, Peck & Greer Venture                151,057           96,481          75,377           12,563          10,302
Associates IV, L.L.C
c/o Weiss, Peck & Greer, LLC
555 California Street, Suite 3130
San Francisco, CA 94104

Weiss, Peck & Greer Venture                 19,003           12,137           9,482            1,580           1,296
Associates IV Cayman, L.P.
c/o Weiss, Peck & Greer, LLC
555 California Street, Suite 3130
San Francisco, CA 94104

David S. Utterberg                         201,410          128,642         753,769          753,769         343,195
c/o Medisystems Corporation
1201 Third Avenue, 39th Floor
Seattle, Washington 98101-3016

Atlas Venture Fund III, L.P.               164,268          104,920          72,536          192,783         118,490
890 Winter Street, Suite 320
Waltham, MA 02451

Atlas Venture Entrepreneurs'                 3,572            2,281           1,577            4,192           2,576
Fund III, L.P.
890 Winter Street, Suite 320
Waltham, MA 02451

Atlas Venture Fund V, L.P.                     -0-              -0-       1,133,073          424,146         260,692
890 Winter Street, Suite 320
Waltham, MA 02451

Atlas Venture Parallel Fund V-A,               -0-              -0-         140,746           52,686          32,382
C.V.
890 Winter Street, Suite 320
Waltham, MA 02451
</TABLE>

                                      -15-
<PAGE>

<TABLE>
<S>                                      <C>              <C>             <C>              <C>             <C>
Atlas Venture Parallel Fund V-B,               -0-              -0-         140,746           52,686          32,382
C.V.
890 Winter Street, Suite 320
Waltham, MA 02451

Atlas Venture Entrepreneurs'                   -0-              -0-          18,860            7,060           4,339
Fund V L.P.
890 Winter Street, Suite 320
Waltham, MA 02451

Marubeni Corporation                           -0-              -0-             -0-              -0-         370,880
4-2, Ohtemachi 1-chome
Chiyoda-ku
Tokyo, Japan

Marubeni America Corporation                   -0-              -0-             -0-              -0-          41,208
450 Lexington Avenue
New York, New York 10017

Wasatch Funds, Inc. for Wasatch                -0-              -0-             -0-              -0-         206,044
Ultra Growth Fund
c/o Wasatch Ultra Growth Fund
150 Social Hall Avenue
Suite 400
Salt Lake City, Utah 84111

CSFB Fund Co-Investment                        -0-              -0-             -0-              -0-         274,725
Program, L.P.
11 Madison Avenue, 16th Floor
New York, NY 10010

Paul Brown                                     -0-              -0-          16,750           12,563          13,736
115 Arbor Court
Woodside, CA 94062

TOTAL                                    1,875,000        1,151,632       4,857,622        2,669,908       2,747,253
</TABLE>

                                      -16-
<PAGE>

                              NxSTAGE MEDICAL, INC.

                             FIFTH AMENDMENT TO THE
                            INVESTOR RIGHTS AGREEMENT

         THIS FIFTH AMENDMENT TO THE INVESTOR RIGHTS AGREEMENT (this "Fifth
Amendment") is entered into as of the 23rd day of December 2004, by and among
NxSTAGE MEDICAL, INC., a Delaware corporation (the "Company"), and certain of
the purchasers of the Company's Series B Preferred Stock, par value $0.001 per
share (the "Series B Preferred Stock"), Series C Preferred Stock, par value
$0.001 per share (the "Series C Preferred Stock"), Series D Preferred Stock, par
value $0.001 per share (the "Series D Preferred Stock"), Series E Preferred
Stock, par value $0.001 per share (the "Series E Preferred Stock"), and Series F
Preferred Stock, par value $0.001 per share (the "Series F Preferred Stock";
such purchasers of the Company's preferred stock are collectively referred to
herein as the "Preferred Stock Investors"), and holders of the Lighthouse
Warrants (as hereinafter defined), all as set forth on the signature pages
hereto.

                                    RECITALS

         WHEREAS, certain of the Preferred Stock Investors purchased, in the
aggregate, 1,875,000 shares of Series B Preferred Stock pursuant to the Series B
Preferred Stock Purchase Agreement dated as of June 30, 1999 (the "Series B
Purchase Agreement");

         WHEREAS, in connection with the acquisition of the Series B Preferred
Stock, the Company and certain of the Preferred Stock Investors entered into an
Investor Rights Agreement dated as of June 30, 1999 (the "Investor Rights
Agreement");

         WHEREAS, certain of the Preferred Stock Investors purchased, in the
aggregate, 1,151,632 shares of Series C Preferred Stock pursuant to the Series C
Preferred Stock Purchase Agreement dated as of January 24, 2001 ("the Series C
Purchase Agreement");

         WHEREAS, in connection with the acquisition of the Series C Preferred
Stock, the Company and certain of the Preferred Stock Investors entered into a
First Amendment to the Investor Rights Agreement dated as of January 24, 2000
(the "First Amendment");

         WHEREAS, certain of the Preferred Stock Investors purchased, in the
aggregate, 4,857,622 shares of Series D Preferred Stock pursuant to the Series D
Preferred Stock Purchase Agreement dated as of May 24, 2001 (the "Series D
Purchase Agreement");

         WHEREAS, in connection with the acquisition of the Series D Preferred
Stock, the Company and certain Preferred Stock Investors entered into a Second
Amendment to the Investor Rights Agreement dated as of May 24, 2001 (the "Second
Amendment");

         WHEREAS, certain of the Preferred Stock Investors purchased, in the
aggregate, 2,669,908 shares of Series E Preferred Stock pursuant to the Series E
Preferred Stock Purchase Agreement dated as of April 15, 2003 (the "Series E
Purchase Agreement");

<PAGE>

         WHEREAS, in connection with the acquisition of the Series E Preferred
Stock, the Company and certain Preferred Stock Investors entered into a Third
Amendment to the Investor Rights Agreement dated as of April 15, 2003 (the
"Third Amendment");

         WHEREAS, certain of the Preferred Stock Investors purchased, in the
aggregate, 2,747,253 shares of Series F Preferred Stock pursuant to the Series F
Preferred Stock Purchase Agreement dated as of August 18, 2004 (the "Series F
Purchase Agreement");

         WHEREAS, in connection with the acquisition of the Series F Preferred
Stock, the Company and certain Investors entered into a Fourth Amendment to the
Investor Rights Agreement dated as of August 18, 2004 (the "Fourth Amendment");
and

         WHEREAS, the Company is seeking to enter into a Loan and Security
Agreement, dated as of the date hereof (the "Lighthouse Loan Agreement"), with
Lighthouse Capital Partners V and/or its affiliated entity, Lighthouse Capital
Partners IV (collectively, the "Lighthouse Entities"), and it is a condition to
the closing of the transactions contemplated by the Lighthouse Loan Agreement
that the Company grant to the Lighthouse Entities certain warrants (the
"Lighthouse Warrants") to purchase shares of Series F Preferred Stock, and
grant, with respect to the Series F Preferred Stock, certain registration rights
and other rights as set forth in the Investor Rights Agreement, as amended by
the First, Second, Third and Fourth Amendments;

         WHEREAS, the Investors executing this Fifth Amendment are the holders
of at least 66 2/3 % of the Registrable Securities, as defined in the Investor
Rights Agreement, as amended.

         NOW THEREFORE, in consideration of the mutual promises,
representations, warranties, covenants and conditions set forth herein and in
the Lighthouse Loan Agreement, the parties hereby agree as follows:

         1.   FOURTH AMENDMENT TO THE INVESTOR RIGHTS AGREEMENT. In accordance
with Section 5.5 of the Investor Rights Agreement, as amended by the First,
Second, Third and Fourth Amendments, the Preferred Stock Investors and the
Company hereby agree to amend the Investor Rights Agreement as follows:

              a. Section 1 of the Investor Rights Agreement is amended to add a
new definition of "Investors" to read in its entirety as follows:

"INVESTORS" shall mean (i) the Preferred Stock Investors and (ii) the holders of
the shares of Series F Preferred Stock or other shares of capital stock issued
upon exercise of the Lighthouse Warrants, but with respect to this subsection
(ii), only after such time that the Lighthouse Warrants have been exercised.

              b. The definition of "Shares" set forth in Section 1 of the
Investor Rights Agreement is amended to read in its entirety as follows:

                                      -2-
<PAGE>

"SHARES" means the Company's Series B Preferred Stock issued pursuant to the
Series B Purchase Agreement, the Company's Series C Preferred Stock issued
pursuant to the Series C Purchase Agreement, the Company's Series D Preferred
Stock issued pursuant to the Series D Purchase Agreement, the Company's Series E
Preferred Stock issued pursuant to the Series E Purchase Agreement and the
Company's Series F Preferred Stock issued (i) pursuant to the Series F Purchase
Agreement, or (ii) upon exercise of the Lighthouse Warrants.

         c. All references to "Investors" and "Shares" throughout the Investor
Rights Agreement shall be read to refer to the amended definitions of
"Investors" and "Shares" as set forth above.

         d. Section 4.1 of the Agreement is amended to read in its entirety as
follows:

                  4.1 SUBSEQUENT OFFERINGS.

                        (a) Subject to Section 4.1(b) below, each Investor shall
                  have a right of first refusal to purchase its Adjusted Pro
                  Rata Share (as defined below) of all Equity Securities that
                  the Company may, from time to time, propose to sell and issue
                  after the date of this Agreement, other than the Equity
                  Securities excluded by Section 4.6 hereof. For purposes
                  hereof, pro rata share shall mean the quotient of the holder's
                  total number of issued and outstanding shares in the
                  Corporation (Preferred and Common) on a fully converted basis
                  (but excluding shares issuable upon exercise of the Lighthouse
                  Warrants), divided by the total number of issued and
                  outstanding shares in the Corporation (Preferred and Common)
                  on a fully converted basis (but excluding shares issuable upon
                  exercise of the Lighthouse Warrants). Each Investor's Adjusted
                  Pro Rata Share, shall be equal to its pro rata share,
                  multiplied by a fraction, the numerator of which is the actual
                  number of shares of Series F Preferred Stock held by such
                  Investor, and the denominator of which is the shares of Series
                  F Preferred Stock associated with the Investor's name in the
                  following table (in no event shall the resultant fraction
                  exceed 100%, and if the Investor is not listed below, the
                  Investor's Adjusted Pro Rata share shall equal zero):

<TABLE>
<S>                                     <C>
Sprout Group                            643,134
WPG                                     111,301
David Utterberg                         230,562
</TABLE>

                                      -3-
<PAGE>

<TABLE>
<S>                                     <C>
Atlas Ventures                          302,892
Adams Street                            102,470
Paul Brown                                3,297
Marubeni                                412,088
CSFB                                    274,725
Wasatch                                 206,044
</TABLE>

                        (b) The Lighthouse Entities, and/or their registered
                  assigns, to the extent such Lighthouse Entities and/or their
                  registered assigns are holding the Lighthouse Warrants, shall
                  have a right of first refusal to purchase $500,000 of Equity
                  Securities issued by the Company in the Company's first equity
                  financing transaction after August 18, 2004 (the "Next Round
                  Financing"), provided that this right shall terminate and be
                  of no further force and effect (i) on the effective date of
                  the registration statement pertaining to an Initial Offering,
                  (ii) from and after the date that the Lighthouse Warrants
                  cease to be outstanding for any reason, or (iii) upon the
                  consummation of the first equity financing transaction after
                  August 18, 2004.

                  e. Section 4.2 of the Agreement is amended to read in its
entirety as follows:

                        4.2 EXERCISE OF RIGHTS: If the Company proposes to issue
                  any Equity Securities, it shall give each Investor written
                  notice of its intention, describing the Equity Securities, the
                  price and the terms and conditions upon which the Company
                  proposes to issue the same. Each Investor shall have fifteen
                  (15) days from the giving of such notice to agree to purchase
                  (i) his, her or its Adjusted Pro Rata share of the Equity
                  Securities for the price and upon the terms and conditions
                  specified in the notice, or (ii) in the case of the holders of
                  the Lighthouse Warrants and in the context of the Next Round
                  Financing, $500,000, collectively, of such Equity Securities,
                  by giving written notice to the Company and stating therein
                  the quantity of Equity Securities to be purchased.
                  Notwithstanding the foregoing, the Company shall not be
                  required to offer or sell such Equity Securities to any
                  Investor who would cause the Company to be in violation of
                  applicable federal securities laws by virtue of such offer or
                  sale.

                  f. Section 4.3 of the Agreement is amended to read in its
entirety as follows:

                        4.3 ISSUANCE OF EQUITY SECURITIES TO OTHER PERSONS: If
                  not all of the Investors elect to purchase their Adjusted Pro
                  Rata Share of such Equity Securities, or in the case of the
                  holders of the

                                      -4-
<PAGE>

                  Lighthouse Warrants and in the context of the Next Round
                  Financing, $500,000 of such Equity Securities, then the
                  Company shall promptly notify in writing the Investors who do
                  so elect and shall offer them the right to acquire their pro
                  rata shares of such unsubscribed shares. Such Investors shall
                  have five (5) days after receipt of such notice to notify the
                  Company of his, her or its election to purchase all or a
                  portion thereof of the unsubscribed shares. If the Investors
                  so notified fail to exercise in full the rights of first
                  refusal, the Company shall have ninety (90) days thereafter to
                  sell the Equity Securities in respect of which the Investors'
                  rights were not exercised, at a price and upon general terms
                  and conditions materially no more favorable to the purchasers
                  thereof than specified in the Company's notice to the
                  Investors pursuant to Section 4.2 hereof. If the Company has
                  not sold such Equity Securities within ninety (90) days of the
                  notice provided pursuant to Section 4.2, the Company shall not
                  thereafter issue or sell any Equity Securities, without first
                  offering such securities to the Investors in the manner
                  provided above.

                  g. Section 4.4 of the Agreement is amended to read in its
entirety as follows:

                        4.4 TERMINATION OF RIGHTS OF FIRST REFUSAL: The rights
                  of first refusal established in Section 4.l(a) shall terminate
                  upon the effective date of the registration statement
                  pertaining to a Qualified Public Offering. The rights of first
                  refusal established in Section 4.1(b) shall terminate as set
                  forth therein.

                  h. Section 4.6 of the Agreement is amended to add a new
Section (h) to read in its entirety as follows:

                        "(h) Any Equity Securities issued to the Lighthouse
                  Entities in connection the Lighthouse Loan Agreement,
                  including the Lighthouse Warrants and the shares of capital
                  stock of the Company issuable upon exercise of the Lighthouse
                  Warrants (the "Warrant Shares") and the shares of capital
                  stock issuable upon conversion of the "Warrant Shares"; and"

                  i. Section 4.6 of the Agreement is amended to rename Section
(h) as Section (i) as follows:

                        "(i) Equity Securities issued in connection with
                  strategic transactions involving the Company and other
                  entities, including (i) joint ventures, manufacturing,
                  marketing or distribution arrangements or (ii) technology
                  transfer or development arrangements; provided that such
                  strategic transactions and the issuance of shares therein, has
                  been approved by not less than

                                      -5-
<PAGE>

                  seventy-five percent (75%) the Company's Board of Directors,
                  with all Directors voting."

                  j. Exhibit A to the Investor Rights Agreement is amended and
restated as set forth on the Exhibit A attached hereto.

                  k. The remainder of the Investor Rights Agreement not amended
hereby remains unchanged and in full force and effect.

      2.    MISCELLANEOUS

            2.1 GOVERNING LAW. This Fifth Amendment is governed by and construed
under the laws of the State of Delaware as applied to agreements among Delaware
residents entered into and to be performed entirely within Delaware.

            2.2 SEVERABILITY. In case any provision of this Fifth Amendment is
determined to be invalid, illegal, or unenforceable, the validity, legality, and
enforceability of the remaining provisions will not in any way be affected or
impaired thereby.

            2.3 TITLES AND SUBTITLES. The titles of the sections of this Fifth
Amendment are for convenience of reference only and are not to be considered in
construing this Agreement.

            2.4 COUNTERPARTS. This Fifth Amendment may be executed in any number
of counterparts, each of which is an original, and all of which together
constitutes one instrument.

         *THE REMAINDER OF THIS PAGE HAS BEEN INTENTIONALLY LEFT BLANK*

                                      -6-
<PAGE>

            IN WITNESS WHEREOF, the parties hereto have executed this Fifth
Amendment to the Investor Rights Agreement as of the date first set forth above.

                                        COMPANY:

                                        NxSTAGE MEDICAL, INC.
                                        439 SOUTH UNION STREET, 5TH FLOOR
                                        LAWRENCE, MA 01843

                                        By:/s/ Jeffrey H. Burbank
                                           -------------------------------------
                                               Name:   Jeffrey H. Burbank
                                               Title:  President

                                        INVESTORS:

                                        /s/ Jeffrey H. Burbank
                                        ----------------------------------------
                                        Jeffrey H. Burbank

                                        /s/ David S. Utterberg
                                        ----------------------------------------
                                        David S. Utterberg

                                        /s/ Paul Brown
                                        ----------------------------------------
                                        Paul Brown

                                      -7-
<PAGE>

                                        MARUBENI CORPORATION

                                        By: /s/ Tetsuji Banno
                                            ------------------------------------
                                                Name:  Tetsuji Banno
                                                Title: General Manager
                                                       Business Incubation Dept.

                                        MARUBENI AMERICA CORPORATION

                                        By: /s/ Shigemasa Sonobe
                                            ------------------------------------
                                                Name:  Shigemasa Sonobe
                                                Title: General Manager
                                                       Investment Business Unit

                                      -8-
<PAGE>

                                        BVCF IV, L.P.

                                        By: Adams Street Partners, LLC
                                            Its General Partner

                                        By: /s/ Craig S. Taylor
                                            ------------------------------------
                                                Name:  Craig S. Taylor
                                                Title: Partner

                                      -9-
<PAGE>

                                        WPG ENTERPRISE FUND III, L.L.C.

                                        By: WPG VC Fund Adviser, L.L.C.
                                            Fund Investment Advisory Member

                                        By: /s/ Dave Markland
                                            ------------------------------------
                                            Name:  Dave Markland
                                            Title: CFOT Administrative Partner

                                        WEISS, PECK & GREER VENTURE
                                        ASSOCIATES IV, L.L.C.

                                        By: WPG VC Fund Adviser, L.L.C.
                                            Fund Investment Advisory Member

                                        By: /s/ [illegible]
                                            ------------------------------------
                                            Name:
                                            Title: Managing Member

                                        WEISS, PECK & GREER VENTURE
                                        ASSOCIATES IV CAYMAN, L.P.

                                        By: WPG VC Fund Adviser, L.L.C.
                                            General Partner

                                        By: /s/ [illegible]
                                            ------------------------------------
                                            Name:
                                            Title: Managing Member

                                      -10-
<PAGE>

                                       ATLAS VENTURE FUND V, L.P.
                                       ATLAS VENTURE PARALLEL FUND V-A, C.V.
                                       ATLAS VENTURE PARALLEL FUND V-B, C.V.
                                       ATLAS VENTURE ENTREPRENEURS' FUND V, L.P.

                                       By:   Atlas Venture Associates V, L.P.
                                             their General Partner

                                       By:   Atlas Venture Associates V, Inc.
                                             Its General Partner

                                       By: /s/ [illegible]
                                           ------------------------------------
                                             Name:
                                             Title:  V.P.

                                       ATLAS VENTURE FUND III, L.P.
                                       ATLAS VENTURE ENTREPRENEURS' FUND
                                       III, L.P.

                                       By:   Atlas Venture Associates III, L.P.
                                             their General Partner

                                       By:   Atlas Venture Associates III, Inc.
                                             Its General Partner

                                       By: /s/ [illegible]
                                           ------------------------------------
                                             Name:
                                             Title:  V.P.

                                      -11-
<PAGE>

                                        SPROUT CAPITAL VIII, L.P.

                                        By:    DLJ Capital Corporation
                                               Its Managing General Partner

                                        By: /s/ Craig Slutzken
                                            ------------------------------------
                                               By:  Craig Slutzken
                                               Its: Vice President

                                        SPROUT VENTURE CAPITAL, L.P.

                                        By:  DLJ Capital Corporation
                                             Its Managing General Partner

                                        By: /s/ Craig Slutzken
                                            ------------------------------------
                                               By:  Craig Slutzken
                                               Its: Vice President

                                        DLJ CAPITAL CORP.

                                        By: /s/ Craig Slutzken
                                            ------------------------------------
                                               By:  Craig Slutzken
                                               Its: Vice President

                                        DLJ CAPITAL CORP.

                                        By: /s/ Craig Slutzken
                                            ------------------------------------
                                               By:  Craig Slutzken
                                               Its: Vice President

                                        DLJ ESC II, L.P.

                                        By: /s/ Craig Slutzken
                                            ------------------------------------
                                               By:  Craig Slutzken
                                               Its: Vice President

                                      -12-
<PAGE>

                                        SPROUT ENTREPRENEURS FUND, L.P.

                                        By:    DLJ Capital Corporation
                                               Its General Partner

                                        By: /s/ Craig Slutzken
                                            ------------------------------------
                                               By:  Craig Slutzken
                                               Its: Vice President

                                        SPROUT CAPITAL IX, L.P.

                                        By:    DLJ Capital Corporation
                                               Its Managing General Partner

                                        By: /s/ Craig Slutzken
                                            ------------------------------------
                                               By:  Craig Slutzken
                                               Its: Vice President

                                        SPROUT IX PLAN INVESTORS, L.P.

                                        By:    DLJ LBO Plans Management
                                               Corporation II
                                               Its General Partner

                                        By: /s/ Craig Slutzken
                                            ------------------------------------
                                               By:  Craig Slutzken
                                               Its: Attorney in Fact

                                        SPROUT PLAN INVESTORS, L.P.

                                        By:    DLJ LBO Plans Management
                                               Corporation II
                                               Its General Partner

                                        By: /s/ Craig Slutzken
                                            ------------------------------------
                                               By:  Craig Slutzken
                                               Its: Attorney in Fact

                                      -13-
<PAGE>

                                        THE SPROUT CEO FUND, L.P.

                                        By:    DLJ Capital Corporation
                                               Its General Partner

                                        By: /s/ Craig Slutzken
                                            ------------------------------------
                                               By:  Craig Slutzken
                                               Its: Vice President

                                        SPROUT CAPITAL VII, L.P.

                                        By:    DLJ Capital Corporation
                                               Its Managing General Partner

                                        By: /s/ Craig Slutzken
                                            ------------------------------------
                                               By:  Craig Slutzken
                                               Its: Vice President

                                      -14-
<PAGE>

                                        WASATCH FUNDS, INC. FOR WASATCH
                                        ULTRA GROWTH FUND

                                        By: /s/ Venice Edwards
                                            ------------------------------------
                                               By:    Venice F. Edwards
                                               Title: Vice President/Treasurer

                                      -15-
<PAGE>

                                        CSFB FUND CO-INVESTMENT PROGRAM, L.P.

                                        By:    DLJ Fund Partners, L.P.
                                               Its General Partner

                                        By:    DLJMB Fund, Inc.
                                               Its General Partner

                                        By: /s/ [illegible]
                                            ------------------------------------
                                               By:
                                               Its: Vice President

                                      -16-
<PAGE>

                                    EXHIBIT A

                                LIST OF INVESTORS

<TABLE>
<CAPTION>
                                                  No. OF          No. OF           No. OF          No. OF          No. OF
                                                SHARES OF       SHARES OF        SHARES OF       SHARES OF       SHARES OF
                                                 SERIES B        SERIES C        SERIES D        SERIES E        SERIES F
                                                PREFERRED       PREFERRED        PREFERRED       PREFERRED       PREFERRED
             NAME AND ADDRESS                     STOCK            STOCK            STOCK          STOCK           STOCK
<S>                                             <C>             <C>              <C>             <C>             <C>
Jeffrey H. Burbank                                74,906             -0-               -0-             -0-             -0-
c/o NxSTAGE MEDICAL, INC.
3 Highwood Drive
Tewksbury, MA 01876

BVCF IV, L.P.                                        -0-             -0-           753,769         157,345          68,681
209 South LaSalle Street
Chicago, IL 60604-1295

Sprout Capital VIII, L.P.                        981,202         628,353               -0-         354,895         447,582
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

Sprout Venture Capital, L.P.                      58,872          37,732               -0-          21,300          26,863
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

DLJ Capital Corp.                                  3,273           2,096            18,910          36,094           6,489
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

DLJ ESC II, L.P.                                  85,383          54,646               -0-          30,756             -0-
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

Sprout Entrepreneurs Fund, L.P.                      -0-             -0-             6,148           1,288           1,624
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

Sprout Capital IX, L.P.                              -0-             -0-         1,559,935         326,824         412,180
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

The Sprout CEO Fund, L.P.                            -0-             -0-               -0-           2,273             -0-
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025
</TABLE>

                                      -17-
<PAGE>

<TABLE>
<CAPTION>
                                                  No. OF          No. OF           No. OF          No. OF          No. OF
                                                SHARES OF       SHARES OF        SHARES OF       SHARES OF       SHARES OF
                                                 SERIES B        SERIES C        SERIES D        SERIES E        SERIES F
                                                PREFERRED       PREFERRED        PREFERRED       PREFERRED       PREFERRED
             NAME AND ADDRESS                     STOCK            STOCK            STOCK          STOCK           STOCK
<S>                                             <C>             <C>              <C>             <C>             <C>
Sprout Capital VII, L.P.                             -0-             -0-              -0-         195,256            -0-
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

Sprout IX Plan Investors                             -0-             -0-           90,049          18,866         23,793
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

Sprout Plan Investors, L.P.                          -0-             -0-              -0-             -0-         38,788
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

WPG Enterprise Fund III, L.L.C.                  132,054          84,344           65,895          10,983          9,006
c/o Weiss, Peck & Greer, LLC
555 California Street, Suite 3130
San Francisco, CA 94104

Weiss, Peck & Greer Venture                      151,057          96,481           75,377          12,563         10,302
Associates IV, L.L.C.
c/o Weiss, Peck & Greer, LLC
555 California Street, Suite 3130
San Francisco, CA 94104

Weiss, Peck & Greer Venture                       19,003          12,137            9,482           1,580          1,296
Associates IV Cayman, L.P.
c/o Weiss, Peck & Greer, LLC
555 California Street, Suite 3130
San Francisco, CA 94104

David S. Utterberg                               201,410         128,642          753,769         753,769        343,195
c/o Medisystems Corporation
1201 Third Avenue, 39th Floor
Seattle, Washington 98101-3016

Atlas Venture Fund III, L.P.                     164,268         104,920           72,536         192,783        118,490
890 Winter Street, Suite 320
Waltham, MA 02451

Atlas Venture Entrepreneurs'                       3,572           2,281            1,577           4,192          2,576
Fund III, L.P.
890 Winter Street, Suite 320
Waltham, MA 02451
</TABLE>

                                      -18-
<PAGE>

<TABLE>
<CAPTION>
                                                  No. OF          No. OF           No. OF          No. OF          No. OF
                                                SHARES OF       SHARES OF        SHARES OF       SHARES OF       SHARES OF
                                                 SERIES B        SERIES C        SERIES D        SERIES E        SERIES F
                                                PREFERRED       PREFERRED        PREFERRED       PREFERRED       PREFERRED
           NAME AND ADDRESS                       STOCK            STOCK            STOCK          STOCK           STOCK
<S>                                             <C>             <C>              <C>             <C>             <C>
Atlas Venture Fund V, L.P.                         -0-             -0-           1,133,073        424,146        260,692
890 Winter Street, Suite 320
Waltham, MA 02451

Atlas Venture Parallel Fund V-A, C.V.              -0-             -0-             140,746         52,686         32,382
890 Winter Street, Suite 320
Waltham, MA 02451

Atlas Venture Parallel Fund V-B, C.V.              -0-             -0-             140,746         52,686         32,382
890 Winter Street, Suite 320
Waltham, MA 02451

Atlas Venture Entrepreneurs' Fund V L.P.           -0-             -0-              18,860          7,060          4,339
890 Winter Street, Suite 320
Waltham, MA 02451

Marubeni Corporation                               -0-             -0-                 -0-            -0-        370,880
4-2, Ohtemachi 1-chome
Chiyoda-ku
Tokyo, Japan

Marubeni America Corporation                       -0-             -0-                 -0-            -0-         41,208
450 Lexington Avenue
New York, New York 10017

Wasatch Funds, Inc. for Wasatch                    -0-             -0-                 -0-            -0-        206,044
Ultra Growth Fund
c/o Wasatch Ultra Growth Fund
150 Social Hall Avenue
Suite 400
Salt Lake City, Utah 84111

CSFB Fund Co-Investment Program, L.P.              -0-             -0-                 -0-            -0-        274,725
11 Madison Avenue, 16th Floor
New York, NY 10010

Paul Brown                                         -0-             -0-              16,750         12,563         13,736
115 Arbor Court
Woodside, CA 94062
</TABLE>

                                      -19-
<PAGE>

<TABLE>
<S>                                             <C>             <C>              <C>             <C>             <C>
Lighthouse Capital Partners IV*                                                                                    [     ]*

Lighthouse Capital Partners V*                                                                                     [     ]*

TOTAL                                           1,875,000       1,151,632        4,857,622       2,669,908       2,747,253
</TABLE>

--------------
*At such time that the Lighthouse Entities or their registered assigns exercise
the Lighthouse Warrants.

                                      -20-

<PAGE>

                              NxSTAGE MEDICAL, INC.

                             SIXTH AMENDMENT TO THE
                            INVESTOR RIGHTS AGREEMENT

      THIS SIXTH AMENDMENT TO THE INVESTOR RIGHTS AGREEMENT (this "Sixth
Amendment") is entered into as of the 8th day of July, 2005, by and among
NxSTAGE MEDICAL, INC., a Delaware corporation (the "Company"), and certain of
the purchasers of the Company's Series B Preferred Stock, par value $0.001 per
share (the "Series B Preferred Stock"), Series C Preferred Stock, par value
$0.001 per share (the "Series C Preferred Stock"), Series D Preferred Stock, par
value $0.001 per share (the "Series D Preferred Stock"), Series E Preferred
Stock, par value $0.001 per share (the "Series E Preferred Stock"), and Series F
Preferred Stock, par value $0.001 per share (the "Series F Preferred Stock"),
and all of the purchasers of the Company's Series F-1 Preferred Stock, par value
$0.001 per share (the "Series F-1 Preferred Stock; such purchasers of the
Company's preferred stock are collectively referred to herein as the "Preferred
Stock Investors"), and holders of the Lighthouse Warrants (as hereinafter
defined), all as set forth on the signature pages hereto.

                                    RECITALS

      WHEREAS, certain of the Preferred Stock Investors purchased, in the
aggregate, 1,875,000 shares of Series B Preferred Stock pursuant to the Series B
Preferred Stock Purchase Agreement dated as of June 30, 1999 (the "Series B
Purchase Agreement");

      WHEREAS, in connection with the acquisition of the Series B Preferred
Stock, the Company and certain of the Preferred Stock Investors entered into an
Investor Rights Agreement dated as of June 30, 1999 (the "Investor Rights
Agreement");

      WHEREAS, certain of the Preferred Stock Investors purchased, in the
aggregate, 1,151,632 shares of Series C Preferred Stock pursuant to the Series C
Preferred Stock Purchase Agreement dated as of January 24, 2000 ("the Series C
Purchase Agreement");

      WHEREAS, in connection with the acquisition of the Series C Preferred
Stock, the Company and certain of the Preferred Stock Investors entered into a
First Amendment to the Investor Rights Agreement dated as of January 24, 2000
(the "First Amendment");

      WHEREAS, certain of the Preferred Stock Investors purchased, in the
aggregate, 4,857,622 shares of Series D Preferred Stock pursuant to the Series D
Preferred Stock Purchase Agreement dated as of May 24, 2001 (the "Series D
Purchase Agreement");

      WHEREAS, in connection with the acquisition of the Series D Preferred
Stock, the Company and certain of the Preferred Stock Investors entered into a
Second Amendment to the Investor Rights Agreement dated as of May 24, 2001 (the
"Second Amendment");

<PAGE>

      WHEREAS, certain of the Preferred Stock Investors purchased, in the
aggregate, 2,669,908 shares of Series E Preferred Stock pursuant to the Series E
Preferred Stock Purchase Agreement dated as of April 15, 2003 (the "Series E
Purchase Agreement");

      WHEREAS, in connection with the acquisition of the Series E Preferred
Stock, the Company and certain Preferred Stock Investors entered into a Third
Amendment to the Investor Rights Agreement dated as of April 15, 2003 (the
"Third Amendment");

      WHEREAS, certain of the Preferred Stock Investors purchased, in the
aggregate, 2,747,253 shares of Series F Preferred Stock pursuant to the Series F
Preferred Stock Purchase Agreement dated as of August 18, 2004 (the "Series F
Purchase Agreement");

      WHEREAS, in connection with the acquisition of the Series F Preferred
Stock, the Company and certain of the Preferred Stock Investors entered into a
Fourth Amendment to the Investor Rights Agreement dated as of August 18, 2004
(the "Fourth Amendment");

      WHEREAS, the Company issued warrants (the "Lighthouse Warrants") to
purchase shares of the Series F Preferred Stock to Lighthouse Capital Partners V
and/or its affiliated entity, Lighthouse Capital Partners IV (collectively, the
"Lighthouse Entities"), pursuant to the Loan and Security Agreement between the
Company and the Lighthouse Entities dated December 23, 2004;

      WHEREAS, in connection with the issuance of the Lighthouse Warrants, the
Company, the Preferred Stock Investors and the Lighthouse Entities entered into
a Fifth Amendment to the Investor Rights Agreement (the "Fifth Amendment"); and

      WHEREAS, the Company and certain of the Preferred Stock Investors (the
"Series F-1 Investors") have entered into a Series F-1 Preferred Stock Purchase
Agreement of even date herewith (the "Series F-1 Purchase Agreement"), pursuant
to which the Series F-1 Investors will purchase the number of shares of Series
F-1 Preferred Stock set forth opposite his or its name on Exhibit A totaling up
to an aggregate of 2,197,801 shares of Series F-1 Preferred Stock; and

      WHEREAS, it is a condition to the closing of the sale of the Series F-1
Preferred Stock, pursuant to the Series F-1 Purchase Agreement, that the Company
grant to the Preferred Stock Investors, with respect to the Series F-1 Preferred
Stock, those registration rights, information rights and other rights as set
forth in the Investor Rights Agreement, as amended to date;

      WHEREAS, the Preferred Stock Investors executing this Sixth Amendment are
the holders of at least 2/3% of the Registrable Securities, as defined in the
Investor Rights Agreement as amended to date.

      NOW THEREFORE, in consideration of the mutual promises, representations,
warranties, covenants and conditions set forth herein and in the F-1 Purchase
Agreement, the parties hereby agree as follows:

1. SIXTH AMENDMENT TO THE INVESTOR RIGHTS AGREEMENT. In accordance with Section
5.5 of the Investor Rights Agreement, as amended by the First, Second, Third,

                                      -2-
<PAGE>

Fourth and Fifth Amendments, the Investors, as defined in the Investor Rights
Agreement, and the Company hereby agree to amend the Investor Rights Agreement
as follows:

      a. The definition of "Shares" set forth in Section 1 of the Investor
Rights Agreement is amended to read in its entirety as follows:

            "SHARES" means the Company's Series B Preferred Stock issued
pursuant to the Series B Purchase Agreement, the Company's Series C Preferred
Stock issued pursuant to the Series C Purchase Agreement, the Company's Series D
Preferred Stock issued pursuant to the Series D Purchase Agreement, the
Company's Series E Preferred Stock issued pursuant to the Series E Purchase
Agreement and the Company's Series F Preferred Stock issued (i) pursuant to the
Series F Purchase Agreement, or (ii) upon exercise of the Lighthouse Warrants,
and the Series F-1 Preferred Stock issued pursuant to the Series F-1 Purchase
Agreement.

      b. All references to "Shares" throughout the Investor Rights Agreement
shall be read to refer to the amended definitions of "Shares" as set forth
above.

      c. Section 2.1(a)(iii) of the Investor Rights Agreement is amended to read
in its entirety as follows:

                  iii. Notwithstanding the provisions of paragraphs (i) and (ii)
above, no such registration statement or opinion of counsel shall be necessary
for a transfer by a Holder which is (A) a partnership to its partners or former
partners in accordance with the partnership interests, (B) a corporation to its
shareholders in accordance with their interests in the corporation, (C) a
limited liability company to its members or former members in accordance with
their interests in the limited liability company, (D) a partnership, corporation
or limited liability company to an affiliate of such entity, or (E) an
individual to the Holder's family member or trust, partnership, limited
liability company or similar entity created solely for the benefit of an
individual Holder and/or such Holder's family members (or from any such trust,
partnership, limited liability company or similar entity to the Holder or its
family member), provided the transferee will be subject to the terms of this
Section 2.1 to the same extent as if such transferee were an original Holder
hereunder. For purposes of this Agreement an affiliate of an Investor shall mean
any person or entity which, directly or indirectly, controls, is controlled by
or is under common control with such Investor, including, without limitation,
any general partner, manager, managing member, officer or director of such
Investor and any investment fund now or hereafter existing which is controlled
by one or more general partners of, or shares the same management as, such
Investor.

      d. Section 2.10 of the Investor Rights Agreement is amended to read in its
entirety as follows:

            ASSIGNMENT OF REGISTRATION RIGHTS. The rights to cause the Company
to register Registrable Securities pursuant to this Section 2 may be assigned by
a Holder to a transferee or assignee of Registrable Securities which (i) is a
subsidiary, parent, general partner, limited partner, retired partner, member or
stockholder of a Holder, (ii) is a Holder's family member or trust, partnership,
limited liability company or similar entity created solely for the benefit of an
individual Holder and/or such Holder's family members, or (iii) acquires at
least one hundred

                                      -3-
<PAGE>

thousand (100,000) shares of Registrable Securities (as presently constituted
and as adjusted for stock splits, reverse stock splits, stock dividends,
recapitalizations, combinations and the like); provided, however, (A) the
transferor shall, within ten (10) days after such transfer, furnish to the
Company written notice of the name and address of such transferee or assignee
and the securities with respect to which such registration rights are being
assigned and (B) such transferee shall agree to be subject to all restrictions
set forth in this Agreement.

      e. Section 2.13 of the Investor Rights Agreement is amended to read in its
entirety as follows:

            "MARKET STAND-OFF" AGREEMENT. If requested by the Company as the
representative of the underwriters of the Initial Offering, each Holder shall
not sell or otherwise dispose of any Shares of Common Stock (or other
securities) of the Company held by each such Holder (other than those included
in the registration) for a period specified by the representatives of the
underwriters not to exceed one hundred eighty (180) days following the effective
date of the registration statement for the Initial Offering, provided that all
officers and directors of the Company and holders of at least one percent (1%)
of the Company's voting securities enter into similar agreements.

      The obligations described in this Section 2.13 shall not apply to a
registration relating solely to employee benefit plans on Form S-3 or Form S-8
or similar forms that may be promulgated in the future relating solely to a
Commission Rule 145 transaction on Form S-4 or similar forms that may be
promulgated in the future. In addition, the obligations described in this
Section 2.13 shall not apply to shares of Common Stock (or other securities) of
the Company purchased by Wasatch Funds, Inc. ("Wasatch") or its affiliates in
connection with or following the Company's Initial Offering. Notwithstanding
anything to the contrary contained herein, the preceding sentence shall not be
amended or waived without the written consent of Wasatch. The Company may impose
stop-transfer instructions with respect to the shares of Common Stock (or other
securities) subject to the foregoing restriction until the end of said one
hundred eighty (180) day period.

      f. Section 4.1 of the Investor Rights Agreement is amended to read in its
entirety as follows:

                  SUBSEQUENT OFFERINGS. Each Investor shall have a right of
first refusal to purchase its Adjusted Pro Rata Share (as defined below) of all
Equity Securities that the Company may, from time to time, propose to sell and
issue after the date hereof, other than the Equity Securities excluded by
Section 4.6 hereof. For purposes hereof, pro rata share shall mean the quotient
of the holder's total number of issued and outstanding shares in the Company
(Preferred and Common) on a fully converted basis (but excluding shares issuable
upon exercise of the Lighthouse Warrants), divided by the total number of issued
and outstanding shares in the Company (Preferred and Common) on a fully
converted basis (but excluding shares issuable upon exercise of the Lighthouse
Warrants). Each Investor's Adjusted Pro Rata Share shall be equal to its pro
rata share, multiplied by a fraction, the numerator of which is the actual
number of shares of Series F-1 Preferred Stock held by such Investor, and the
denominator of which is the shares of Series F-1 Preferred Stock associated with
the Investor's name in the following table (in no event shall the resultant
fraction exceed 100%, and if the Investor is not listed below,

                                      -4-
<PAGE>

the denominator used in calculating Adjusted Pro Rata Share for that Investor
shall equal the number of shares of Series F-1 Preferred Stock held by such
Investor).

<TABLE>
<S>                                              <C>
Sprout Group                                       301,563
WPG                                                 45,636
David Utterberg                                    108,109
Atlas Ventures                                     142,025
Adams Street                                        44,260
CSFB                                                12,410
Wasatch                                              9,308
Marubeni                                            21,556
Paul Brown                                           1,945
Caxton                                           1,373,626
</TABLE>

      g. For purposes of the Investor Rights Agreement, the term "Investor"
shall include (i) a transferee permitted by Section 2.10 of the Investor Rights
Agreement and (ii) purchasers of the Company's Series F-1 Preferred Stock who,
after the date hereof, agree to become a party to and be bound by the Investor
Rights Agreement by executing and delivering to the Company a counterpart
signature page thereto in such form as the Company may designate, and it is
agreed that the Company may admit such persons as Investors party to the
Investor Rights Agreement, as amended.

      h. Exhibit A to the Investor Rights Agreement is amended and restated in
its entirety as set forth on Exhibit A attached hereto.

      i. The remainder of the Investor Rights Agreement as not amended hereby,
remains unchanged and in full force and effect.

2. MISCELLANEOUS

      2.1 GOVERNING LAW. This Sixth Amendment is governed by and construed under
the laws of the State of Delaware as applied to agreements among Delaware
residents entered into and to be performed entirely within Delaware.

      2.2 SEVERABILITY. In case any provision of this Sixth Amendment is
determined to be invalid, illegal or unenforceable, the validity, legality and
enforceability of the remaining provisions will not in any way be affected or
impaired thereby.

      2.3 TITLES AND SUBTITLES. The titles of the sections of this Sixth
Amendment are for convenience of reference only and are not to be considered in
construing this Agreement.

      2.4 COUNTERPARTS. This Sixth Amendment may be executed in any number of
counterparts, each of which is an original, and all of which together
constitutes one instrument.

          THE REMAINDER OF THIS PAGE HAS BEEN INTENTIONALLY LEFT BLANK

                                      -5-
<PAGE>

      IN WITNESS WHEREOF, the parties hereto have executed this Sixth Amendment
to the Investor Rights Agreement as of the date first set forth above.

                                         COMPANY:

                                         NxSTAGE MEDICAL, INC.
                                         439 SOUTH UNION STREET, 5TH FLOOR
                                         LAWRENCE, MA 01843

                                         By:/s/ Jeffrey H. Burbank
                                            ----------------------------------
                                         Name:
                                         Title:

                                         INVESTORS:

                                         /s/ Jeffrey H. Burbank
                                         -------------------------------------
                                         Jeffrey H. Burbank

                                         /s/ David S. Utterberg
                                         -------------------------------------
                                         David S. Utterberg

                                         /s/ Paul Brown
                                         -------------------------------------
                                         Paul Brown

                                      -6-
<PAGE>

                                         HEALTHCARE INVESTMENT PARTNERS
                                         HOLDINGS LLC

                                         By: /s/ Henry Wend
                                             ---------------------------------
                                         Name: Henry Wend
                                         Title: Manager

        [SIGNATURE PAGE TO SIXTH AMENDMENT TO INVESTOR RIGHTS AGREEMENT]

                                      -7-
<PAGE>

                                          CSFB FUND CO-INVESTMENT PROGRAM, L.P.

                                          By: DLJ Fund Partners L.P.
                                              Its General Partner

                                          By: DLJMB Fund, Inc.
                                              Its General Partner

                                          By: /s/ [illegible]
                                              -------------------------------
                                          Name:
                                          Title:

        [SIGNATURE PAGE TO SIXTH AMENDMENT TO INVESTOR RIGHTS AGREEMENT]

                                      -8-
<PAGE>

                                    SPROUT CAPITAL VIII, L.P.

                                    By: DLJ Capital Corporation
                                        Its Managing Partner

                                    By:/s/ Philippe Chambon
                                       ---------------------------------------
                                    Name:  Philippe Chambon
                                    Title: Managing Director

                                    SPROUT VENTURE CAPITAL, L.P.

                                    By: DLJ Capital Corporation
                                        Its General Partner

                                    By:/s/ Philippe Chambon
                                       ---------------------------------------
                                    Name:  Philippe Chambon
                                    Title: Managing Director

                                    DLJ CAPITAL CORP.

                                    By:/s/ Philippe Chambon
                                       ----------------------------------------
                                    Name: Philippe Chambon
                                    Title: Managing Director

                                    DLJ ESC II, L.P.

                                    By: DLJ LBO Plans Management Corporation
                                        Its General Partner

                                    By:/s/ Philippe Chambon
                                       -----------------------------------------
                                    Name:  Philippe Chambon
                                    Title: Attorney in Fact

        [SIGNATURE PAGE TO SIXTH AMENDMENT TO INVESTOR RIGHTS AGREEMENT]

                                      -9-
<PAGE>

                                    SPROUT ENTREPRENEURS FUND, L.P.

                                    By: DLJ Capital Corp.
                                        Its General Partner

                                    By:/s/ Philippe Chambon
                                       ---------------------------------------
                                    Name:  Philippe Chambon
                                    Title: Managing Director

                                    SPROUT CAPITAL IX, L.P.

                                    By: DLJ Capital Corporation
                                        Its Managing General Partner

                                    By:/s/ Philippe Chambon
                                       ---------------------------------------
                                    Name:  Philippe Chambon
                                    Title: Managing Director

                                    SPROUT IX PLAN INVESTORS, L.P.

                                    By: DLJ LBO Plans Management Corporation II
                                        Its General Partner

                                    By:/s/ Philippe Chambon
                                       ---------------------------------------
                                    Name:  Philippe Chambon
                                    Title: Attorney in Fact

                                    SPROUT PLAN INVESTORS L.P.

                                    By: DLJ LBO Plans Management Corporation
                                        Its General Partner

                                    By:/s/ Philippe Chambon
                                       ---------------------------------------
                                    Name:  Philippe Chambon
                                    Title: Attorney in Fact

        [SIGNATURE PAGE TO SIXTH AMENDMENT TO INVESTOR RIGHTS AGREEMENT]

                                      -10-
<PAGE>

                                    THE SPROUT CEO FUND, L.P.

                                    By: DLJ Capital Corporation
                                        Its General Partner

                                    By:/s/ Philippe Chambon
                                       ---------------------------------------
                                    Name:  Philippe Chambon
                                    Title: Managing Director

                                    SPROUT CAPITAL VII, L.P.

                                    By: DLJ Capital Corporation
                                        Its Managing General Partner

                                    By:/s/ Philippe Chambon
                                       ---------------------------------------
                                    Name:  Philippe Chambon
                                    Title: Managing Director

        [SIGNATURE PAGE TO SIXTH AMENDMENT TO INVESTOR RIGHTS AGREEMENT]

                                      -11-
<PAGE>

                                    ATLAS VENTURE FUND V, L.P.
                                    ATLAS VENTURE PARALLEL FUND V-A, C.V.
                                    ATLAS VENTURE PARALLEL FUND V-B, C.V.
                                    ATLAS VENTURE ENTREPRENEURS' FUND V, L.P.

                                    By: Atlas Venture Associates V, L.P.
                                        Their General Partner

                                    By: Atlas Venture Associates V, Inc.
                                        Its General Partner

                                    By:/s/ [illegible]
                                       ---------------------------------------
                                    Name:
                                    Title: V.P.

                                    ATLAS VENTURE FUND III, L.P.
                                    ATLAS VENTURE ENTREPRENEURS' FUND III, L.P.

                                    By: Atlas Venture Associates III, L.P.
                                        their General Partner

                                    By: Atlas Venture Associates III, L.P.
                                        Its General Partner

                                    By:/s/ [illegible]
                                       ---------------------------------------
                                    Name:
                                    Title: V.P.

        [SIGNATURE PAGE TO SIXTH AMENDMENT TO INVESTOR RIGHTS AGREEMENT]

                                      -12-
<PAGE>

                                    MARUBENI CORPORATION

                                    By: /s/ Toshihide Kubo
                                       ---------------------------------------
                                    Name:  Toshihide Kubo
                                    Title: General Manager
                                           Business Incubation Dept.

                                    MARUBENI AMERICA CORPORATION

                                    By:_______________________________________
                                    Name:
                                    Title:

        [SIGNATURE PAGE TO SIXTH AMENDMENT TO INVESTOR RIGHTS AGREEMENT]

                                      -13-
<PAGE>

                                    BVCF IV, L.P.

                                    By: Adams Street Partners, LLC
                                        Its General Partner

                                    By:/s/ Craig S. Taylor
                                       --------------------------------------
                                    Name:  Craig S. Taylor
                                    Title: Partner

        [SIGNATURE PAGE TO SIXTH AMENDMENT TO INVESTOR RIGHTS AGREEMENT]

                                      -14-
<PAGE>

                                    WASATCH FUNDS, INC.
                                    FOR WASATCH SMALL CAP GROWTH FUND

                                    By:/s/ Angela M. Palmer
                                       ---------------------------------------
                                    Name: Angela M. Palmer
                                    Its:  Secretary

                                    WASATCH FUNDS, INC.
                                    FOR WASATCH ULTRA GROWTH FUND

                                    By:/s/ Angela M. Palmer
                                       ---------------------------------------
                                    Name:  Angela M. Palmer
                                    Title: Secretary

        [SIGNATURE PAGE TO SIXTH AMENDMENT TO INVESTOR RIGHTS AGREEMENT]

                                      -15-
<PAGE>

                                    EXHIBIT A

                            LIST OF KEY STOCKHOLDERS

<TABLE>
<CAPTION>
                                                                     OPTION FOR THE
                                              NUMBER OF SHARES         PURCHASE OF             TOTAL COMMON
        NAME AND ADDRESS                      OF COMMON STOCK         COMMON STOCK           STOCK EQUIVALENT
-------------------------------------         ----------------       ---------------         ----------------
<S>                                           <C>                    <C>                     <C>
Dr. C. David Finch                                 243,307                    -0-                 243,307
1828 Raymond Road
Jackson, MI 39204
(601) 373-7897
(601) 373-7899 (Fax)

Dr. Hendrik K. Kuiper                              216,969                 27,491                 244,460
1115 N. Frontage Road
P.O. Box 348
Vicksburg, MI 39180-5102
(601) 634-8790
(601) 619-4121 (Fax)

Jeffrey H. Burbank                                 598,409                330,347                 928,756
18 Sunrise Road
Boxford, MA 01921

A.D. & S.R. Burbank Trust                           37,641                    -0-                  37,641
c/o Jeffrey H. Burbank
18 Sunrise Road
Boxford, MA 01921

J.B. Backcock                                        2,030                    -0-                   2,030
10626-G York Road
Hunt Valley, MD 21030
(410) 666-8500
(410) 666-6267 (Fax)
</TABLE>

                                      -16-
<PAGE>

                                    EXHIBIT A

                                LIST OF INVESTORS

<TABLE>
<CAPTION>
                                          NUMBER OF     NUMBER OF         NUMBER OF      NUMBER OF       NUMBER OF      NUMBER OF
                                          SHARES OF     SHARES OF         SHARES OF      SHARES OF       SHARES OF      SHARES OF
                                          SERIES B      SERIES C          SERIES D       SERIES E        SERIES F       SERIES F-1
                                          PREFERRED     PREFERRED         PREFERRED      PREFERRED       PREFERRED      PREFERRED
           NAME AND ADDRESS                 STOCK         STOCK             STOCK          STOCK           STOCK          STOCK
---------------------------------------   ---------     ---------         ---------      ---------       ---------      ----------
<S>                                       <C>           <C>               <C>            <C>             <C>            <C>
Jeffrey H. Burbank                          74,906           -0-               -0-            -0-              -0-            -0-
18 Sunrise Road
Boxford, Massachusetts 01921

BVCF IV, L.P.                                  -0-           -0-           753,769        157,345           68,681         21,857
c/o Adams Street Partners, LLC
209 South LaSalle Street
Chicago, IL 60604-1295

WPG Enterprise Fund III, L.L.C.            132,054        84,344            65,895         10,983            9,006          2,866
Weiss, Peck & Greer, LLC
555 California Street, Suite 3130
San Francisco, California 94104
Fax: (415) 989-5108

Weiss, Peck & Greer Venture                151,057        96,481            75,377         12,563           10,302          3,279
Associates IV, L.L.C.
c/o Weiss, Peck & Greer, LLC
555 California Street,
Suite 3130
San Francisco, CA 94104
Fax:  (415) 989-5108

Weiss, Peck & Greer Venture                 19,003        12,137             9,482          1,580            1,296            412
Associates IV
Cayman, L.P.
c/o Weiss, Peck & Greer, LLC
555 California Street
Suite 3130
San Francisco, CA 94104
Fax:  (415) 989-5708

David S. Utterberg                         201,410       128,642           753,769        753,769          343,195        109,220
c/o Medisystems Corporation
1201 Third Avenue, 39th Floor
Seattle, Washington 98101-3016
copy to:  John Willett, Esq.
Arnold & Porter
399 Park Avenue
New York, NY 10022-4690
</TABLE>

                                      -17-
<PAGE>

<TABLE>
<S>                                        <C>           <C>              <C>             <C>              <C>            <C>
Atlas Venture Fund III, L.P.               164,268       104,920             72,536       192,783          118,490         37,709
890 Winter Street,
Suite 320
Waltham, MA 02451
Fax: (781) 622-1701

Atlas Venture Entrepreneurs' Fund            3,572         2,281              1,577         4,192            2,576            820
III, L.P.
890 Winter Street,
Suite 320
Waltham, MA 02451
Fax: (781) 622-1701

Atlas Venture Fund V, L.P.                     -0-           -0-          1,133,073       424,146          260,692         82,964
890 Winter Street,
Suite 320
Waltham, MA 02451
Fax: (781) 622-1701

Atlas Venture Parallel Fund V-A C.V.           -0-           -0-            140,746        52,686           32,382         10,305
890 Winter Street,
Suite 320
Waltham, MA 02451
Fax: (781) 622-1701

Atlas Venture Parallel Fund V-B C.V.           -0-           -0-            140,746        52,686           32,382         10,305
890 Winter Street, Suite 320
Waltham, MA 02451
Fax: (781) 622-1701

Atlas Venture Entrepreneurs'                   -0-           -0-             18,860         7,060            4,339          1,381
Fund V, L.P.
890 Winter Street,
Suite 320
Waltham, MA 02451
Fax: (781) 622-1701

Sprout Capital VIII, L.P.                  981,202       628,353                -0-       354,895          447,582        142,440
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

Sprout Venture Capital, L.P.                58,872        37,732                -0-        21,300           26,863          8,549
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

DLJ Capital Corp.                            3,273         2,096             18,910        36,094            6,489          2,065
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025
</TABLE>

                                      -18-
<PAGE>

<TABLE>
<S>                                        <C>            <C>             <C>             <C>              <C>            <C>
DLJ ESC II, L.P.                           85,383         54,646                -0-        30,756              -0-            -0-
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

Sprout Capital VII, L.P.                      -0-            -0-                -0-       195,256              -0-            -0-
3000 Sand Hill road
Building 3, Suite 170
Menlo Park, California 94025

Sprout Entrepreneurs Fund, L.P.               -0-            -0-              6,148         1,288            1,624            517
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

The Sprout CEO Fund, L.P.                     -0-            -0-                -0-         2,273              -0-            -0-
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

Sprout Capital IX, L.P.                       -0-            -0-          1,559,935       326,824          412,180        131,174
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

Sprout IX Plan Investors, L.P.                -0-            -0-             90,049        18,866           23,793          7,572
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

Sprout Plan Investors, L.P.                   -0-            -0-                -0-           -0-           38,788         12,344
3000 Sand Hill Road
Building 3, Suite 170
Menlo Park, California 94025

Paul Brown                                    -0-            -0-             16,750        12,563           13,736          4,371
115 Arbor Court
Woodside, CA 94062

Marubeni Corporation                          -0-            -0-                -0-           -0-          370,880            -0-
4-2, Ohtemachi 1-chome
Chiyoda-ku
Tokyo, Japan

Marubeni America Corporation                  -0-            -0-                -0-           -0-           41,208            -0-
450 Lexington Avenue
New York, New York 10017
</TABLE>

                                      -19-
<PAGE>

<TABLE>
<S>                                       <C>           <C>               <C>            <C>              <C>              <C>
Wasatch                                         -0-           -0-               -0-            -0-          206,044           -0-
Ultra Growth Fund
c/o Wasatch Ultra Growth Fund
150 Social Hall Avenue
Suite 400
Salt Lake City, Utah 84111

Wasatch Small Cap Growth Fund                   -0-           -0-               -0-            -0-              -0-        65,572
c/o Wasatch Small Cap Growth Fund
150 Social Hall Avenue Suite 400
Salt Lake City, Utah 84111

CSFB Fund Co-Investment Program, L.P.           -0-           -0-               -0-            -0-          274,725        31,090
11 Madison Avenue, 16th Floor
New York, NY 10010

Lighthouse Capital Partners IV*                                                                              [    ]*          -0-

Lighthouse Capital Partners V*                                                                               [    ]*          -0-

TOTAL                                     1,875,000     1,151,632         4,857,622      2,669,908        2,747,253
</TABLE>

                                      -20-<PAGE>
                                                                   EXHIBIT 10.10

                                                              FROM THE OFFICE OF
                                                         Heritage Place LLC. c/o
                                                        Ozzy Property Mgmt, Inc.
                                                         3 Dundee Office Pk, B05
                                                               Andover, MA 01810

                         STANDARD FORM COMMERCIAL LEASE

1. PARTIES                    LESSOR, which expression shall include Heritage
(fill in)                     Place, LLC, heirs, successors, and assigns where
                              the context so admits, does hereby lease to
                              NxSTAGE Medical, Inc., 3 Highwood Drive Tewksbury,
                              MA 01876

2. PREMISES                   LESSEE, which expression shall include NxSTAGE
(fill in and include, if      Medical, Inc., successors, executors,
applicable, suite number,     administrators, and assigns where the context so
floor number, and square      admits, and the LESSEE hereby leases the following
feet)                         described premises:

                              Approximately 44,050 rentable square feet
                              consisting of 43,050 rentable square feet on the
                              fifth floor in Heritage Place, Lawrence, MA, of
                              which 26,139 RSF is finished space and 16,911 RSF
                              will be unfinished in accordance with Base
                              Building descriptions below. Approximately 1,000
                              rentable square feet will be basement storage
                              space. The leased premises are shown by
                              cross-hatching on the plan of the building
                              attached hereto as Exhibit "A" together with the
                              right to use in common with others entitled
                              thereto, the hallways, the stairways, and
                              elevators necessary for access to said leased
                              premises nearest thereto and the right to use a
                              portion of the roof for HVAC installation and
                              maintenance, provided that such use will not in
                              any way interfere with the LESSOR's right to
                              utilize the roof for other purposes including, but
                              not limited to, leasing such roof area to wireless
                              communications facilities.

                              together with the right to use in common, with
                              others entitled thereto, the hallways, stairways,
                              and elevators, necessary for access to said leased
                              premises, and lavatories nearest thereto.

3. TERM                       The term of this lease shall be for five years
(fill in)                     commencing on see paragraph 52       and ending on

4. RENT                       The LESSEE shall pay to the LESSOR fixed rent at
(fill in)                     the rate of 422, 736 dollars per year, payable in
                              advance in monthly installments of $ 35, 228
                              subject to proration in the case of any partial
                              calendar month. All rent shall be payable without
                              offset or deduction.

                              continued in paragraph 33
<PAGE>

5. SECURITY DEPOSIT           Upon the execution of this lease, the LESSEE shall
 (fill in)                    pay to the LESSOR the amount of 450, 000 dollars,
                              which shall be held as a security for the LESSEE'S
                              performance as herein provided and refunded to the
                              LESSEE at the end of this lease subject to the
                              LESSEE's satisfactory compliance with the
                              conditions hereof.

                              continued in paragraph 47

6. RENT ADJUSTMENT            If in any tax year commencing with the fiscal year
                              2002 , the real estate taxes on the land and
                              buildings, of which the leased premises are a
   A.  TAX ESCALATION         part, are in excess of the amount of the real
      (fill in or delete)     estate taxes thereon for the fiscal year 2001
                              (hereinafter called the "Base Year"), LESSEE will
                              pay to LESSOR as additional rent hereunder, when
                              and as designated by notice in writing by LESSOR,
                              14.68 per cent of such excess that may occur in
                              each year of the term of this lease or any
                              extension or renewal thereof and proportionately
                              for any part of a fiscal year. If the LESSOR
                              obtains an abatement of any such excess real
                              estate tax, a proportionate share of such
                              abatement, less the reasonable fees and costs
                              incurred in obtaining the same, if any, shall be
                              refunded to the LESSEE.

   B.  OPERATING COST         The LESSEE shall pay to the LESSOR as additional
       ESCALATION             rent hereunder when and as designated by notice in
       (fill in or delete)    writing by LESSOR, 14.68 per cent of any increase
                              in operating expenses over those incurred during
                              the calendar year 2001. Operating expenses are
                              defined for the purposes of this agreement as all
                              costs and expenses incurred by the LESSOR during
                              any calendar year in connection with the operation
                              and maintenance of the land and buildings of which
                              the leased premises are a part, including without
                              limitation insurance premiums, license fees,
                              janitorial service, landscaping and snow removal,
                              employee compensation and fringe benefits,
                              equipment and materials, utility costs, repairs,
                              maintenance and any capital expenditure
                              (reasonably amortized with interest) incurred in
                              order to reduce other operating expenses or comply
                              with any governmental requirement.

                              Continued in paragraph 48

                              This increase shall be prorated should this lease
                              be in effect with respect to only a portion of any
                              calendar year.

7. UTILITIES                  The LESSEE shall pay, as they become due, all
delete                        bills for electricity and other utilities (whether
"air conditioning"            they are used for furnishing heat or other
if not applicable             purposes) that are furnished to the leased
                              premises and presently separately metered, and all
                              bills for fuel furnished to a separate tank
                              servicing the leased premises exclusively. The

                                                                          Page 2
<PAGE>

                              LESSOR agrees to provide all other utility service
                              and to furnish reasonably hot and cold water and
                              reasonable heat and air conditioning (except to
                              the extent that the same are furnished through
                              separately metered utilities or separate fuel
                              tanks as set forth above) to the leased premises,
                              the hallways, stairways, elevators, and lavatories
                              during normal business hours on regular business
                              days of the heating and air conditioning seasons
                              of each year, to furnish elevator service and to
                              light passageways and stairways during business
                              hours, and to furnish such cleaning service as is
                              customary in similar buildings in said city or
                              town, all subject to interruption due to any
                              accident, to the making of repairs, alterations,
                              or improvements, to labor difficulties, to trouble
                              in obtaining fuel, electricity, service, or
                              supplies from the sources from which they are
                              usually obtained for said building, or to any
                              cause beyond the LESSOR's control.

                              LESSOR shall have no obligation to provide
                              utilities or equipment other than the utilities
                              and equipment within the premises as of the
                              commencement date of this lease. In the event
                              LESSEE requires additional utilities or equipment,
                              the installation and maintenance thereof shall be
                              the LESSEE's sole obligation, provided that such
                              installation shall be subject to, the written
                              consent of the LESSOR. continued in addendum
                              paragraph 28

8. USE OF LEASED PREMISES     The LESSEE shall use the leased premises only for
(fill in)                     the purpose of professional office space,
                              laboratory, light manufacturing and warehouse
                              space. except as provided for in paragraph 37

9. COMPLIANCE WITH LAWS       The LESSEE acknowledges that no trade or
                              occupation shall be conducted in the leased
                              premises or use made thereof which will be
                              unlawful, improper, noisy or offensive, or
                              contrary to any law or any municipal by-law or
                              ordinance in force in the city or town in which
                              the premises are situated. Without limiting the
                              generality of the foregoing (a) the LESSEE shall
                              not bring or permit to be brought or kept in or on
                              the leased premises or elsewhere on the LESSOR's
                              property any hazardous, toxic, inflammable,
                              combustible or explosive fluid, material, chemical
                              or substance, including without limitation any
                              item defined as hazardous pursuant to Chapter 21 E
                              of the Massachusetts General Laws; and (b) the
                              LESSEE shall be responsible for compliance with
                              requirements imposed by the Americans with
                              Disabilities Act relative to the layout of the
                              leased premises and any work performed by the
                              LESSEE therein continued in paragraph 55

                                                                          Page 3
<PAGE>

10. FIRE INSURANCE            The LESSEE shall not permit any use of the leased
                              premises which will make voidable any insurance on
                              the property of which the leased premises are a
                              part, or on the contents of said property or which
                              shall be contrary to any law or regulation from
                              time to time established by the New England Fire
                              Insurance Rating Association, or any similar body
                              succeeding to its powers. The LESSEE shall on
                              demand reimburse the LESSOR, and all other
                              tenants, all extra insurance premiums caused by
                              the LESSEE's use of the premises.

                              with reasonable wear and tear,

11. MAINTENANCE               The LESSEE agrees to maintain the leased premises
                              in good condition, damage by fire and other
                              casualty only excepted, and whenever necessary, to
   A. LESSEE'S OBLIGATIONS    replace plate glass and other glass therein,
                              acknowledging that the leased premises are now in
                              good order and the glass whole. The LESSEE shall
                              not permit the leased premises to be overloaded,
                              damaged, stripped, or defaced, nor suffer any
                              waste. LESSEE shall obtain written consent of
                              LESSOR before erecting any sign on the premises.

   B. LESSOR'S OBLIGATIONS    The LESSOR agrees to maintain the structure of the
                              building of which the leased premises are a part
                              in the same condition as it is at the commencement
                              of the term or as it may be put in during the term
                              of this lease, reasonable wear and tear, damage by
                              fire and other casualty only excepted, unless such
                              maintenance is required because of the LESSEE or
                              those for whose conduct the LESSEE is legally
                              responsible.

12. ALTERATIONS - ADDITIONS   The LESSEE shall not make structural alterations
                              or additions to the leased premises, but may make
                              non-structural alterations provided the LESSOR
                              consents thereto in writing, which consent shall
                              not be unreasonably withheld or delayed. All such
                              allowed alterations shall be at LESSEE's expense
                              and shall be in quality at least equal to the
                              present construction. LESSEE shall not permit any
                              mechanics' liens, or similar liens, to remain upon
                              the leased premises for labor and material
                              furnished to LESSEE or claimed to have been
                              furnished to LESSEE in connection with work of any
                              character performed or claimed to have been
                              performed at the direction of LESSEE and shall
                              cause any such lien to be released of record
                              forthwith without cost to LESSOR. Any alterations
                              or improvements made by the LESSEE shall become
                              the property of the LESSOR at the termination of
                              occupancy as provided herein. continued in
                              addendum paragraph 49

13. ASSIGNMENT                The LESSEE shall not assign or sublet the whole or
                              any part of the leased premises without LESSOR's
                              prior written consent.

                                                                          Page 4
<PAGE>

SUBLEASING                    Notwithstanding such consent, LESSEE shall remain
                              liable to LESSOR for the payment of all rent and
                              for the full performance of the covenants and
                              conditions of this lease.

14. SUBORDINATION             This lease shall be subject and subordinate to any
                              and all mortgages, deeds of trust and other
                              instruments in the nature of a mortgage, now or at
                              any time hereafter, a lien or liens on the
                              property of which the leased premises are a part
                              and the LESSEE shall, when requested, promptly
                              execute and deliver such written instruments as
                              shall be necessary to show the subordination of
                              this lease to said mortgages, deeds of trust or
                              other such instruments in the nature of a
                              mortgage.

                              continued in addendum paragraph 50

15. LESSOR'S ACCESS           The LESSOR or agents of the LESSOR may, at
                              reasonable times, enter to view the leased
                              premises and may remove placards and signs not
                              approved and affixed as herein provided, and make
                              repairs and alterations as LESSOR should elect to
                              do and may show the leased premises to others, and
                              at any time within three (3) months before the
                              expiration of the term, may affix to any suitable
                              part of the leased premises a notice for letting
                              or selling the leased premises or property of
                              which the leased premises are a part and keep the
                              same so affixed without hindrance or molestation.

                              continued in addendum paragraph 56

16. INDEMNIFICATION AND       The LESSEE shall save the LESSOR harmless from all
LIABILITY                     loss and damage occasioned by anything occurring
(fill in)                     on the leased premises unless caused by the
                              negligence or misconduct of the LESSOR, and from
                              all loss and damage wherever occurring occasioned
                              by any omission, fault, neglect or other
                              misconduct of the LESSEE. The removal of snow and
                              ice from the sidewalks bordering upon the leased
                              premises shall be LESSOR' S responsibility.

17. LESSEE'S LIABILITY        The LESSEE shall maintain with respect to the
INSURANCE                     leased premises and the property of which the
(fill in)                     leased premises are a part comprehensive public
                              liability insurance in the amount of $1,000,000
                              with property damage insurance in limits of
                              $1,000,000 in responsible companies qualified to
                              do business in Massachusetts and in good standing
                              therein insuring the LESSOR as well as LESSEE
                              against injury to persons or damage to property as
                              provided. The LESSEE shall deposit with the LESSOR
                              certificates for such insurance at or prior to the
                              commencement of the term, and thereafter within
                              thirty (30) days prior to the expiration of any
                              such policies. All such insurance

                                                                          Page 5
<PAGE>

                              certificates shall provide that such policies
                              shall not be cancelled without at least ten (10)
                              days prior written notice to each assured named
                              therein.

18. FIRE, CASUALTY - EMINENT  Should a substantial portion of the leased
DOMAIN                        premises, or of the property of which they are a
                              part, be substantially damaged by fire or other
                              casualty, or be taken by eminent domain, the
                              LESSOR may elect to terminate this lease. When
                              such fire, casualty, or taking renders the leased
                              premises substantially unsuitable for their
                              intended use, a just and proportionate abatement
                              of rent shall be made, and the LESSEE may elect to
                              terminate this lease if:

                              (a) The LESSOR fails to give written notice within
                              thirty (30) days of intention to restore leased
                              premises, or

                              (b) The LESSOR fails to restore the leased
                              premises to a condition substantially suitable for
                              their intended use within ninety (90) days of said
                              fire, casualty or taking.

                              The LESSOR reserves, and the LESSEE grants to the
                              LESSOR, all rights which the LESSEE may have for
                              damages or injury to the leased premises for any
                              taking by eminent domain, except for damage to the
                              LESSEE's fixtures, property, or equipment.

19. DEFAULT AND BANKRUPTCY    In the event that:
(fill in)

                              (a) The LESSEE shall default in the payment of any
                              installment of rent or other sum herein specified
                              and such default shall continue for ten (10) days
                              after written notice thereof; or

                              (b) The LESSEE shall default in the observance or
                              performance of any other of the LESSEE's
                              covenants, agreements, or obligations hereunder
                              and such default shall not be corrected within
                              thirty (30) days after written notice thereof; or
                              (continued in addendum paragraph 51)

                              (c) The LESSEE shall be declared bankrupt or
                              insolvent according to law, or, if any assignment
                              shall be made of LESSEE's property for the benefit
                              of creditors,

                              then the LESSOR shall have the right thereafter,
                              while such default continues, to re-enter and take
                              complete possession of the leased premises, to
                              declare the term of this lease ended, and remove
                              the LESSEE's effects, without prejudice to any
                              remedies which might be otherwise used for arrears
                              of rent or other default. The LESSEE shall
                              indemnify the LESSOR against all loss of rent and
                              other payments which the LESSOR may incur by
                              reason of such termination during the residue of
                              the term. If the LESSEE

                                                                          Page 6
<PAGE>

                              shall default, after reasonable notice thereof, in
                              the observance or performance of any conditions or
                              covenants on LESSEE's part to be observed or
                              performed under or by virtue of any of the
                              provisions in any article of this lease, the
                              LESSOR, without being under any obligation to do
                              so and without thereby waiving such default, may
                              remedy such default for the account and at the
                              expense of the LESSEE. If the LESSOR makes any
                              expenditures or incurs any obligations for the
                              payment of money in connection therewith,
                              including but not limited to, reasonable
                              attorney's fees in instituting, prosecuting or
                              defending any action or proceeding, such sums paid
                              or obligations insured, with interest at the rate
                              of 18 per cent per annum and costs, shall be paid
                              to the LESSOR by the LESSEE as additional rent.

20. NOTICE                    Any notice from the LESSOR to the LESSEE relating
(fill in)                     to the leased premises or to the occupancy
                              thereof, shall be deemed duly served if mailed to
                              the leased premises, registered or certified mail,
                              return receipt requested, postage prepaid,
                              addressed to the LESSEE. Any notice from the
                              LESSEE to the LESSOR relating to the leased
                              premises or to the occupancy thereof, shall be
                              deemed duly served, if mailed to the LESSOR by
                              registered or certified mail, return receipt
                              requested, postage prepaid, addressed to the
                              LESSOR at such address as the LESSOR may from time
                              to time advise in writing. All rent notices shall
                              be paid and sent to the LESSOR at Heritage Place,
                              LLC c/of Ozzy Property Management, Inc., 3 Dundee

                              Office Park, Suite B05, Andover, MA 01810

21. SURRENDER                 The LESSEE shall at the expiration or other
                              termination of this lease remove all LESSEE's
                              goods and effects from the leased premises,
                              (including, without hereby limiting the generality
                              of the foregoing, all signs and lettering affixed
                              or painted by the LESSEE, either inside or outside
                              the leased premises). LESSEE shall deliver to the
                              LESSOR the leased premises and all keys, locks
                              thereto, and other fixtures connected therewith
                              and all alterations and additions made to or upon
                              the leased premises, in good condition, damage by
                              fire or other casualty only excepted. In the event
                              of the LESSEE's failure to remove any of LESSEE's
                              property from the premises, LESSOR is hereby
                              authorized, without liability to LESSEE for loss
                              or damage thereto, and at the sole risk of LESSEE,
                              to remove and store any of the property at
                              LESSEE's expense, or to retain same under LESSOR's
                              control or to sell at public or private sale,
                              without notice any or all of the property not so
                              removed and to apply the net proceeds of such sale
                              to the payment of any sum due hereunder, or to
                              destroy such

                                                                          Page 7
<PAGE>

                              property.

22. CONDITION OF              in paragraph 32 of the addendum & on Exhibits B&C
PREMISES                      attached hereto

                              Except as may be otherwise expressly set forth
                              herein, the LESSEE shall accept the leased
                              premises "as is" in their condition as of the
                              commencement of the term of this lease, and the
                              LESSOR shall be obligated to perform no work
                              whatsoever in order to prepare the leased premises
                              for occupancy by the LESSEE.

23. FORCE MAJEURE             In the event that the LESSOR is prevented or
                              delayed from making any repairs or performing any
                              other covenant hereunder by reason of any cause
                              reasonably beyond the control of the LESSOR, the
                              LESSOR shall not be liable to the LESSEE therefor
                              nor, except as expressly otherwise provided in
                              case of casualty or taking, shall the LESSEE be
                              entitled to any abatement or reduction of rent by
                              reason thereof, nor shall the same give rise to a
                              claim by the LESSEE that such failure constitutes
                              actual or constructive eviction from the leased
                              premises or any part thereof.

24. LIABILITY OF OWNER        No owner of the property of which the leased
                              premises are a part shall be liable hereunder
                              except for breaches of the LESSOR's obligations
                              occurring during the period of such ownership. The
                              obligations of the LESSOR shall be binding upon
                              the LESSOR's interest in said property, but not
                              upon other assets of the LESSOR, and no individual
                              partner, agent, trustee, stockholder, officer,
                              director, employee or beneficiary of the LESSOR
                              shall be personally liable for performance of the
                              LESSOR's obligations hereunder.

25. OTHER PROVISIONS          It is also understood and agreed that

                              The addendum attached hereto is hereby
                              incorporated by reference into this lease.

IN WITNESS WHEREOF, the said parties hereunto set their hands and seals this
17th day of October, 2000

/s/ Jeffrey H. Burbank                          /s/ Orit Goldstien

                                                LESSOR    Heritage Place, LLC

                                                                          Page 8
<PAGE>

LESSEE                                                       Member

_________________________________             ________________________________
LESSEE                                        LESSOR

                      _________________________________
                      BROKERS

                                                                          Page 9
<PAGE>

                      STANDARD ADDENDUM TO COMMERCIAL LEASE
                               FOR HERITAGE PLACE

26. UTILITIES. The terms and provisions as set forth in this Paragraph 28 are
intended to be incorporated by reference into Paragraph 7 of the Lease. LESSEE
shall arrange and be solely responsible for janitorial, cleaning and security
services on the leased premises. LESSEE shall provide and pay for it's own
telephone and/or cable services. LESSOR acknowledges and agrees that LESSEE may
need to install additional ventilating and other air treatment systems within
the leased premises or as allowed on the roof under P 2 above in connection with
the construction by LESSEE of a "clean room" within the leased premises. LESSOR
hereby pre-approves and consents to such installation if such additional systems
are deemed necessary by LESSEE provided that such use shall not interfere with
any other tenants right to peaceable use and occupation of their own units.

27. COMPLIANCE WITH LAWS. LESSEE acknowledges and agrees that LESSOR has made no
representation or warranty as to the compliance of LESSEE's proposed use of the
leased premises (as described in Section 8 of the Lease) with any zoning or
other approvals required by governmental authorities in connection with LESSEE's
specific use of the leased premises shall be obtained by LESSEE at LESSEE's sole
cost and expense. After initial construction to be completed by LESSOR meeting
all applicable codes and unless such work is initiated at the request of LESSOR
or other tenants in the building, LESSEE shall be solely responsible for
correcting any failure to comply with any requirements of the Americans with
Disabilities Act or the regulations of the Massachusetts Architectural Access
Board, which failure is result of LESSEE's particular use (including without
limitation any use which shall result in the characterization of the leased
premises as a "public accommodation" or "place of public accommodation") of the
leased premises for other than professional office uses as permitted under
section 8 of this Lease.

28. OPTION TO EXTEND ON TENANCY-AT-WILL BASIS. Provided that this Lease is then
in full force and effect and provided that LESSEE is not then in default
hereunder, the parties may extend this Lease on 30-day tenancy-at-will basis,
upon all the terms, conditions and provisions of this Lease at the expiration of
the term, as the same may be extended as herein provided. If the parties elect
to so extend the term of this Lease, either party may thereafter terminate the
tenancy-at-will, with or without cause by thirty (30) days' written notice to
the other party.

29. BROKERS. LESSOR and LESSEE warrant and represent to each other that no agent
or broker was involved on their behalf in the negotiation or consummation of
this lease (other than CRF Partners and Auburndale Realty whose commissions
shall be paid by LESSOR upon lease commencement). LESSOR and LESSEE each agree
to indemnify and hold harmless the other from any and all claims, brokerage
commissions or fees arising out of any communications or negotiations between
LESSOR or LESSEE, as the case may be, and any broker or agent regarding the
leased premises or consummation of this Lease (other than that commission
identified in this Section 31).

30. PREMISES "AS IS". LESSEE acknowledges that it has inspected the leased
premises and agreed to accept the leased premised in their "AS IS" condition,
subject only to those

                                                                         Page 10
<PAGE>

improvements or alterations identified on Exhibit B and Exhibit C attached
hereto, and incorporated into this Lease and described in Section 49 hereof.

31. RENT. LESSEE shall pay the rent described in Section 4 on the first of each
month, without set off, abatement or reduction. Any payment of rent received by
LESSOR more than ten days after the same is due shall be subject to an
administrative charge of 5% of the amount of such overdue payment. Payments of
rent made more than fifteen days after the date due shall incur an additional
charge of 1% per month until the same is paid in full. LESSEE and LESSOR agree
that the rent as set forth in Section 4 shall be effective for a period of one
year from the commencement date of the term of this Lease (as set forth in
Section 3), and shall be subject to annual increases as follows, such increases
to become effective on the respective anniversary of the commencement date of
the term of this lease:

<TABLE>
<S>         <C>              <C>            <C>              <C>
Year 2:     $444,262.50      per year;      $37,022.00       per month

Year 3:     $465,787.50      per year;      $38,816.00       per month

Year 4:     $487,312.50      per year;      $40,609.00       per month

Year 5:     $508,837.50      per year;      $42,403.00       per month
</TABLE>

Notwithstanding the foregoing, LESSOR agrees that the rent increases set forth
above for Lease Years 4 and 5 shall be subject to and shall not be effective
until completion of Lessor's improvements to the land and the building specified
on Exhibit E hereof.

32. PARTNERSHIP. If LESSEE is a partnership, then LESSEE and LESSOR agree that:
(a) the liability of each of the parties comprising LESSEE shall be joint and
several; (b) each of the parties comprising lessee hereby consents and agrees to
be bound by any written instrument executed by any one of such parties
comprising LESSEE which may hereafter be executed which modifies or discharges
this Lease; (c) any bills, notices, or other communications given or rendered to
LESSEE or any of the parties comprising LESSEE shall be deemed given or rendered
to LESSEE and to all such parties and shall be binding upon LESSEE and all such
parties; (d) if LESSEE shall admit new partners, all of such new partners shall,
by their admission to LESSEE, be deemed to have assumed performance of all of
the terms, covenants and conditions of the Lease on LESSEE's part to be observed
and performed; (e) LESSEE shall give prompt notice to LESSOR of the admission of
any such new partners, and upon demand of LESSOR, shall cause each such new
partner to execute and deliver to LESSOR an agreement in form satisfactory to
LESSOR evidencing such new partner's assumption of the obligations described in
(d) above.

33. PARKING. LESSEE shall be entitled to parking spaces, 3 per 1,000 square feet
of finished rentable space. At the beginning of the lease term, this represents
78 spaces. LESSEE's right to utilize such parking spaces shall be on a
nonexclusive and unassigned basis. Per the provisions of Section 12 herein,
Lessee will notify Lessor of the completion of any non-finished space, upon
which Lessor will insure additional parking spaces are allotted and available
within the terms set forth above.

                                                                         Page 11
<PAGE>

34. ALTERATIONS. In addition to LESSEE's obligations under Section 21, LESSEE
shall, if requested in writing by LESSOR at the time of Lessor's grant of
consent to such alterations or improvements, prior to the expiration or earlier
termination of this Lease, remove alterations or improvements made by LESSEE to
the leased premises, repair any damage resulting from such removal, and restore
the leased premises to the conditions in which it existed prior to such
alteration (except for ordinary wear and tear). Such removal shall be
conditioned upon LESSEE not being in default under the terms and conditions of
the Lease at the time of such removal and, further, contingent upon LESSEE
providing LESSOR with satisfactory assurances of LESSEE's ability to indemnify
LESSOR for any cost or expense incurred by LESSOR including, but not limited to,
repair of damages resulting from such removal. Such evidence may include, at
LESSOR's discretion, the posting of an additional security deposit.

35. HAZARDOUS MATERIALS. (I) LESSEE shall not keep in the leased premises any
inflammable fluids or chemicals, or permit the emission from the leased premises
of any objectionable noise or odor; nor dump, flush, or in any way introduce any
hazardous substances or any other toxic substances into the sewage or other
waste disposal system serving the leased premises or the building; nor generate,
store, use or dispose of hazardous or toxic substances in or on the leased
premises, the building or the office park in which they are located except as
specifically allowed under the terms and conditions hereof, and, in any event,
at all times in conformance with applicable laws, regulations, and ordinances.
LESSEE shall indemnify, defend and hold harmless LESSOR from and against any
damages, suits, costs or expenses resulting from LESSEE's failure to comply with
the covenants contained in this Section 37. For the purposes of the covenants
contained herein this Section 37, LESSEE is intended and does specifically
include all of LESSEE'S agents, employees, licensees, invitees, guests entering
onto the leased premises or anywhere within Heritage Place. "Hazardous
substances" and "toxic substances" as used herein shall have the same meanings
as defined and used in the Comprehensive Environmental Response, Compensation
and Liability Act of 1980, as amended, 42 U.S.C. 9061 et seq., in the Hazardous
Material Transportation Act, 49 U.S.C. 1802; in the Toxic Substances Act, 15
U.S.C. 2601 et seq.; in the Resource Conservation and Recovery Act, as amended,
42 U.S.C. 6921 et seq.; in the Massachusetts Hazardous Waste Management Act, as
amended; in Massachusetts General Laws Chapter 21; in Massachusetts Oil and
Hazardous Material Release Prevention and Response Act, as amended; in the
regulations adopted and publications promulgated pursuant to said Acts; and in
any other applicable laws, rules, regulations and orders.

(II) Notwithstanding the aforementioned, LESSEE has specifically requested that
it be allowed to use the Premises for certain activities which involve the use
of various chemicals, as more fully set forth on Exhibit F hereto. LESSOR, being
unfamiliar with said chemicals, has solely entered into this Lease Agreement
based upon LESSEE's obligation herein to obtain the consent and approval of an
environmental engineer for purposes of use of said chemicals within the leased
premises. LESSEE shall submit the list, attached hereto as Exhibit F, to an
environmental engineer as set out on Exhibit G hereto. LESSEE may retain any
other licensed and reputable environmental engineering, subject to LESSOR's
reasonable consent, which shall not be unreasonably withheld, conditioned or
delayed. Said environmental engineer shall review the chemicals and processes to
make a determination as to whether or not the use thereof within the leased
premises would violate section (1), Paragraph 37, of this Lease Agreement. If
the environmental engineer finds a violation, which remains unremedied beyond
any applicable cure

                                                                         Page 12
<PAGE>

periods, then based upon the recommendations of the environmental engineer,
LESSOR shall have the option to:

      (a)   Require LESSEE to immediately cease the activity which constitutes
            such violation,

      (b)   require LESSEE to install such devices or other protective measures
            as may be recommended by the environmental engineer (including but
            not limited to devices which would deal directly with the chemicals
            as well as with respect to any odors or fumes produced thereby)
            prior to commencing its occupancy, or

      (c)   allow the LESSEE to commence its occupancy as contemplated herein.

The review to be conducted by the environmental engineer, as well as any and all
remediation as advised, shall be conducted prior to the LESSEE's occupancy
hereunder. To the extent of any change of circumstances whatsoever relating to
the chemicals, processes, or other environmental conditions, LESSOR shall have
the right to require an updated environmental review. All such review and
remediation shall be at LESSEE's sole cost and expense. It shall be LESSEE's
affirmative obligation to immediately notify LESSOR prior to any proposed change
in chemicals and/or processes, which change shall immediately necessitate a
review by the environmental engineer, at LESSEE's sole cost and expense, and
subject to the provisions hereof.

36. CERTIFICATES. LESSEE agrees to execute and deliver to LESSOR, within five
days of LESSOR's request therefore, an estoppels certificate in the form
requested by LESSOR attesting to the status of rent payments, security deposits
held by LESSOR, the continued effectiveness and enforceability of the Lease, and
the absence of any defaults on the part of LESSOR (or indicating the nature of
any defaults known to LESSEE), and such other matters relating to this lease or
the leased premises as LESSOR may reasonably request.

37. RESERVED RIGHTS. LESSOR shall have the right to place in the leased premises
(but in such manner as to reduce to a minimum interference with LESSEE's use of
the leased premises), utility lines, equipment, stacks, shafts, pipes, conduits,
ducts and the like.

38. MASTER KEY SYSTEM. All exterior doors will be part of Heritage Place Master
Key System.

39. SIGNAGE. Lessor will provide the same signage to Lessee as it has to other
tenants located within Heritage Place.

40. LESSOR BUILDOUT. Refer to Exhibit "B" and Exhibit "C".

41. ASSIGNMENT AND SUBLETTING. Notwithstanding the provisions of Section 13
herein, LESSEE may assign or sublease the whole or any part of the leased
premises without the consent of LESSOR, provided that such assignment or
sublease is to a wholly-owned subsidiary or affiliate of LESSEE whose use of the
leased premises is consistent with the terms of Paragraph 8 herein. Otherwise,
LESSEE may assign or sublease the whole or any part of the leased premises only
with the prior written consent of the LESSOR, which consent shall not be
unreasonably withheld or delayed, provided the assignee's or sublessee's use of
the leased premises is

                                                                         Page 13
<PAGE>

consistent with typical office building use and zoning requirements for the
Building. LESSEE shall have the right to retain the profits, if any, derived
from any assignment or sublease.

42. RENEWAL OPTION.

      a)    Provided that, at the time of such exercise, (I) this lease is in
            full force and effect and LESSEE continues to occupy the leased
            premises and (ii) LESSEE is not then in default hereunder, LESSEE
            shall have the right and option to extend the Term of the lease for
            one (1) extended term (the "Extended Term" ) of five (5) years by
            giving written notice to LESSOR not later than six (6) months nor
            earlier than twelve (12) months prior to the expiration date of the
            original Term. The effective giving of such notice of extension by
            LESSEE shall automatically extend the Term of this Lease for the
            Extended Term, and no instrument of renewal or extension need be
            executed. In the event that LESSEE fails timely to give such notice
            to LESSOR, this Lease shall automatically terminate at the end or
            the original Term and LESSEE shall have no further option to extend
            the Term of this Lease. The Extended Term shall commence on the day
            immediately succeeding the expiration date of the original Term and
            shall end on the day immediately preceding the fifth (5th)
            anniversary of the first day of such Extended Term. The Extended
            Term shall be on all the terms and conditions of the Lease, except:
            (I) during the Extended Term, LESSEE shall have no further option to
            extend the Term and (ii) the Rent for the Extended Term shall be the
            Fair Market Rental Value of the leased premises as of the
            commencement of the Extended Term in question, taking into account
            all relevant factors, determined pursuant to paragraph (b) below,
            but in no event shall the Rent for the Extended Term be less than
            the Rent payable with respect to the last twelve (12) months of the
            original Term.

      b)    Promptly after receiving LESSEE's notice extending the Term of this
            Lease pursuant to paragraph (a) above, LESSOR shall provide LESSEE
            with LESSOR's good faith estimate of the Fair Market Rental Value of
            the leased premises for the Extended Term based upon rents being
            paid by tenants renewing leases in the area in which the Property is
            located. If LESSEE is unwilling to accept LESSOR's estimate of the
            Fair Market Rental Value as set forth in LESSOR's notice referred to
            above, and the parties are unable to reach agreement thereon within
            thirty (30) days after the delivery of such notice by LESSOR, then
            Lessee shall have the right to rescind its notice extending the Term
            and the Lease Term shall expire in accordance the terms hereof. If
            Lessee does not exercise such right of rescission, then LESSOR and
            LESSEE shall, not later than thirty (30) days after the expiration
            of the aforesaid thirty (30) day period, each retain a real estate
            professional with at least ten (10) years continuous experience in
            the business of appraising or marketing commercial real estate in
            the greater Lawrence area who shall, within thirty (30) days of his
            or her selection, prepare a written report summarizing his or her
            determination of the Fair Market Rental Value for such Extended
            Term. LESSOR and LESSEE shall simultaneously exchange such reports;
            provided, however, if either party has not obtained such a report
            within thirty (30) days after LESSEE receives LESSOR'S notice, then
            the determination

                                                                         Page 14
<PAGE>

            set forth in the other party's report shall be final and binding
            upon the parties. If both parties receive reports within such time,
            but LESSOR and LESSEE cannot within ten (10) days agree upon the
            Fair Market Rental Value, then LESSOR and- LESSEE shall mutually
            select a person with the qualifications stated above (the "Final
            Professional") to resolve the dispute as to the Fair Market Rental
            Value for such Extended Term. If LESSOR and LESSEE cannot agree upon
            the designation of the Final Professional within thirty (30) days of
            the exchange of the first valuation reports, either party may apply
            to the American Arbitration Association, the Greater Boston Real
            Estate Board, or any successor thereto, for the designation of a
            Final Professional. LESSOR and LESSEE shall each promptly submit to
            the Final Professional a copy of their respective real estate
            professional's determination of the Fair Market Rental Value for
            such Extended Term. The Final Professional shall not perform his or
            her own valuation, but rather shall, within thirty (30) days after
            such submissions, select the submission which is closest to the
            determination of the Fair Market Rental Value for such Extended Term
            which the Final Professional would have made acting alone. The Final
            Professional shall give notice of his or her selection to LESSOR and
            LESSEE and such decision shall be final and binding upon LESSOR and
            LESSEE, provided, however, in no event shall the Fair Market Rental
            Value be less than the annual Rent in effect immediately preceding
            such Extended Term. Each party shall pay the fees and expenses of
            its real estate professional and counsel, if any, in connection with
            any proceeding under this paragraph, and the parties shall each pay
            one-half of the fees and expenses of the Final Professional.

43. BUILDING SECURITY. LESSOR shall provide security services in the common
areas of the Building and parking areas from 1:00 p.m. to 10:00 p.m. ("guard
hours") on regular business days. Before and after guard hours on business days,
access to security services shall be provided to LESSEE upon telephonic request
to a security number provided by LESSOR, whereupon LESSOR's security service
shall respond to the premises within a reasonable period of time to provide
escort to the parking lot or to respond to other security concerns. LESSOR shall
have no obligation to provide security on weekends or holidays. LESSEE shall be
solely responsible for all security systems within the leased premises.

44. FINANCIAL STATEMENTS. No more than twice per year, LESSOR may request
updated and audited financial statements and business plans from LESSEE. LESSEE
shall provide said documents within ten (10) business days.

45. SECURITY DEPOSIT. LESSEE acknowledges and agrees to pay the sum of
$450,000.00 as a security deposit for its performance under the terns and
provisions hereof. Such amount shall be held at Andover Bank, or such other bank
as LESSOR sees fit, in an interest bearing account, for the benefit of LESSEE
(LESSEE's taxpayer identification number is 04-3454702). Commencing on January
01, 2003, or at any time thereafter, if and to the extent that LESSEE can show
pretax profit for two consecutive calendar quarters, based upon financial
statements audited by Arthur Anderson, then on a month to month basis, LESSOR
shall, and LESSEE authorizes LESSOR to, apply the security deposit directly to
rent and additional rent accruing under the terms and provisions of the Lease.
LESSOR shall have no obligation to apply the security deposit to the rent or
additional rent at any time when there exists an event of default

                                                                         Page 15
<PAGE>

beyond applicable cure periods. Further, in the event that LESSEE has previously
committed material defaults under the Lease such that LESSOR has resorted to
commencement of litigation or other eviction proceedings, then LESSOR's
obligation to apply the security deposit hereunder shall be terminated. In the
absence of any prior default, LESSOR's obligation to apply the security deposit
hereunder, including any interest which may have accrued to LESSEE's benefit,
shall continue until such time as the balance remaining in the security deposit
account equals the last month's rent, or $42,403.00. At such time, LESSOR shall
be entitled to close the security deposit account, and transfer the balance
thereof to LESSOR's general operating accounts, continue to carry such amount on
it books and records, and shall be entitled to commingle such finds.

46. CONTINUED FROM P 6 SECTION B, OPERATING COST ESCALATION. Operating expenses
shall not include any costs incurred in connection with Lessor's rehabilitation
of the building and improvements at Heritage Place or Lessor's performance of
Base Building work in the premises or the building; Lessee shall have the right
to examine and audit Lessor's books and records establishing operating expenses.
An audit shall consist of review of on site documents evidencing expenses
incurred relating to operations, and no other records. In addition, prior to
disclosing any such information, the LESSEE shall agree to be bound by a
confidentiality agreement recognizing that the information may not be disclosed
to any other party.

47. CONTINUED FROM P 12. ALTERATIONS-ADDITIONS. Notwithstanding the foregoing,
or anything contained herein to the contrary, all articles of personal property
and all business fixtures, machinery and equipment and furniture owned or
installed by Lessee solely at its expense in the leased premises, including
without limitation Lessee's warehouse racking system, but excluding HVAC,
lighting, electrical and plumbing, shall remain the property of the Lessee and
may be removed by Lessee at any time prior to the expiration or earlier
termination of this lease, provided that Lessee, at its expense, shall repair
any damage to the building caused by such removal. In addition, at any time at
which the LESSEE is in default under the Lease, they shall not be entitled to
remove any of the above items without LESSOR's consent which may be withheld
pending satisfactory evidence of LESSEE's ability to restore premises to its
original condition..

48. CONTINUED FROM P 14, SUBORDINATION. Provided that the Mortgagee shall agree
to enter into non-disturbance and attornment agreements, simultaneous with the
subordination agreement, upon conventional terms and provisions which are
standard for such Mortgagee.

49. CONTINUED FROM P 19 SECTION B, DEFAULT AND BANKRUPTCY. [, if such default
cannot reasonably be corrected within such 30 days, if Lessee does not commence
correction within such 30 days and diligently prosecute same to completion, but
in any event no more than ninety (90) days, or]

50. COMMENCEMENT DATE. The Commencement Date shall be the last to occur of
[February 15, 2000] or the day following the Substantial Completion Date. For
purposes of this Lease, the Substantial Date shall be

      (i)   the first day as of which Lessor's Work has been completed except
            for items of work (and, if applicable, adjustment of equipment and
            fixtures) which can be

                                                                         Page 16
<PAGE>

            completed after occupancy has been taken without causing undue
            interference with Lessee's use of the premises (i.e. so-called
            "punch list" items),

      (ii)  Lessee has been given notice thereof, and,

      (iii) Lessee has been afforded an opportunity to walk through the building
            to inspect Lessor's Work and has received certification from
            Lessor's architect that Lessor's Work has been completed in
            accordance with the plans and specifications referenced on Exhibit B
            attached hereto and the Base Building requirements referenced on
            Exhibit C attached hereto.

      (iv)  LESSOR has secured a Certificate of Occupancy for and on behalf of
            LESSEE. Further to the extent that the Certificate of Occupancy is
            withheld for any of LESSEE's work as shown on Exhibit B, LESSOR
            shall be deemed to have satisfied this provision.

Regardless of the requirements set forth in (i) - (iv) above, LESSEE recognizes
that certain work to be performed by LESSOR, as identified as LESSOR's Work
herein or otherwise, will require certain cooperation by LESSEE. In addition,
LESSEE acknowledges and agrees that it is performing certain work, identified as
LESSEE's Work and as more fully set forth on Exhibit B hereto. In cooperating
with LESSOR's Work and performing LESSEE's Work hereunder, LESSEE agrees to
promptly and diligently perform all of its obligations in a commercially
reasonable manner, and to take any and all steps as may be necessary to ensure
that LESSEE's failure to perform the same does not in any way interfere with
LESSOR's ability to complete LESSOR's Work hereunder or to obtain the
Certificate of Occupancy. In the event that LESSEE fails to perform hereunder,
any and all delays caused as a result thereof, shall automatically provide
LESSOR with an extension of the Construction Deadline and any and all deadlines
hereunder this Lease for a duration equal to LESSEE's delay. Where LESSEE's
default hereunder results in delays to LESSOR's ability to substantially
complete LESSOR's Work, or, to obtain the Certificate of Occupancy, the
Substantial Completion Date shall be deemed to be five (5) business days from
the date that LESSOR could have satisfied subparagraph (i) above had LESSEE not
defaulted hereunder. In addition, LESSEE's failure to perform hereunder, in a
prompt and diligent manner, shall also constitute a material default under the
terms and conditions of the Lease.

Lessor shall complete as soon as conditions permit all "punch list" items, and
Lessee shall afford Lessor access to the premises for such purposes. Promptly
upon the occurrence of the Commencement Date, Lessor and Lessee shall execute a
letter substantially in the form attached hereto as Exhibit D, but the failure
by either party to execute such a letter shall have no effect on the
Commencement Date, as hereinabove determined. If the Substantial Completion Date
has not occurred by February 15, 2001 (the "Construction Deadline"), Lessee
shall have the right to terminate this Lease by giving notice to Lessor not
later than thirty (30) days after the Construction Deadline of Lessee's election
so to do, and Lessor shall have 30 days to cure such default. Failing which,
this Lease shall cease and come to an end without further liability or
obligation on the part of either parting after the giving of such notice, except
Lessor's obligation to immediately rebate in full any security deposit delivered
to Lessor by Lessee as provided for herein. Such right of termination shall be
Lessee's sole and exclusive remedy at law or in equity

                                                                         Page 17
<PAGE>

for Lessor's failure so to complete such Lessor's Work on or before the
Construction Deadline. The Construction Deadline shall automatically be extended
for the period of any delays caused by Force Majeure or for any delay caused by
Lessee.

51. TERM. The Term of this Lease shall commence on the Commencement Date and
expire at the close of the day immediately preceding the fifth (5th) anniversary
of the Commencement Date, except that if the Commencement Date is other than the
first day of a calendar month, the expiration of the term shall be at the close
of the last day of the calendar month in which such anniversary falls. The Term
shall include any extension thereof that is expressly provided for by this Lease
and that is affected strictly in accordance with this Lease.

52. LESSOR'S WORK. Lessor, at its sole cost and expense, shall perform the
Tenant Leasehold Improvements and the Base Building Work specified on Exhibits B
and C attached hereto. Lessor agrees to reasonably cooperate with Lessee to
allow Lessee to enter the premises and install Lessee's fixtures, equipment and
furnishings prior to the Commencement Date and during the performance of the
Lessor's Work, provided, however that such entry and installation by Lessee
shall not materially and unreasonably interfere with the performance of Lessor's
Work. Lessee shall give Lessor notice, not later than six months after the
Commencement Date, of any respects in which Lessor has not performed Lessor's
Work fully, properly and in accordance with the terms of this Lease and Lessor
shall remedy same within thirty (30) days after receipt of such notice. To the
extent such default can not be remedied within thirty (30) days, Lessor shall be
provided a commercially reasonable period of time, so long as Lessor continues
to diligently pursue such remedy.

53. CONTINUED FROM P 9. SECTION (B). after initial construction to be completed
by LESSOR meeting all applicable codes and unless such work is initiated at the
request of LESSOR or other tenants in the Building.

54. CONTINUED FROM P 15. LINE ONE. AFTER "TIMES". and except in the case of
emergency with reasonable notice and with minimal interference with LESSEE's
business operations.

The parties hereto acknowledge the inclusion of the Addendum into the Lease.

/s/ Jeffrey H. Burbank                   /s/ Orit Goldstien
--------------------------------         ------------------------------------
Lessee                                   Heritage Place, LLC
                                         Lessor

                                                                         Page 18
<PAGE>

                                    EXHIBIT B

Buildout not to exceed construction estimate by Dowgiert Construction. The
buildout for NxSTAGE Medical, Inc. includes all specifications created by GSD
Associates dated August 10, 2000. Specific references to the following special
rooms will be built out as follows:

Clean Room:             Walls will be dry-walled on exterior with foil back
                        drywall. Power feed will also be installed to this area.
                        Any additional work required in the clean rooms will be
                        handled by MI Systems and is not included in this price.

Tech Lab:               To be built out to standard office finish. Standard sink
                        and plumbing is included.

Wash Room:              Built to same specifications as Clean Room. Plumbing and
                        automatic hand washing is included.

All cabinets will be standard wood or laminate type (not custom made) with post
form laminate tops. No cages, furniture, or special equipment specifically
needed by NxSTAGE are included unless shown on the above-mentioned plan such as
compressor, sinks, and emergency eye wash.

                                                                         Page 19
<PAGE>

                                    EXHIBIT C

                       LESSOR'S BASE BUILDING IMPROVEMENTS

As part of the base building improvements to be part of the entire leased space,
Lessor will:

      1.    Install new HVAC systems for the entire fifth floor that will meet
            all codes for typical office use.

      2.    Replace all exterior windows on the floor.

      3.    Sandblast and clean all interior surfaces.

      4.    New passenger elevator ordered by lease commencement date.

      5.    Installation of at least one new passenger elevator to the 5th floor
            within 12 months of the Commencement Date.

      6.    Construct new code compliant M/W bathrooms.

      7.    Provide 24-Hour repair service for the freight elevator.

      8.    Fire sprinkler system distributed throughout the premises, code
            compliant per the space plan.

      9.    Window blinds in place.

      10.   All emergency/exit lighting per code.

      11.   Typical office electrical use in office areas, additional voltage of
            220 and 440 availability in Lab and Controlled environments.

      12.   Electrical service to the unfinished space distributed to the
            Premises including main and sub panels.

      13.   Main AC duct into unfinished space w/ AC chillers and equipment in
            place, HVAC boxes in place and grid ducted throughout and ready for
            distribution.

As part of the Lessee's building improvements included at Lessor's expense:

      1.    Interior Doors will be solid core

      2.    An allowance of $1/RSF for the finished space for ceiling treatments

      3.    An allowance for $18 Sq. yard for floor covering

      4.    An allowance of $2.50/RSF for lighting systems

As part of the base building improvements to be part of the common leased space,
Lessor will

      1.    New hallway in building one.

      2.    New tile/carpet in public corridors.

      3.    Reseal/Restripe and relamp/relight parking lot

      4.    Reseal/Reshape to be completed on or before May 31, 2001.

                                                                         Page 20
<PAGE>

                                    EXHIBIT D

                            COMMENCEMENT DATE LETTER

                                                  ________________________, 2001

[Name of Contact]
[Name of Tenant]
[Address of Tenant]

      RE:   [Name of Tenant]
            [Premises Rentable Area and Floor]
            [Address of Building]

Dear [Name of Contact]:

      Reference is made to that certain Lease, dated as of       , 2000, between
[Landlord], as Landlord and [Tenant] as Tenant, with respect to Premises on the
floor of the above-referenced building. In accordance with Section ____________
of the Addendum to Lease, this is to confirm that the Commencement Date of the
Term of the Lease occurred on____________, and that the Term of the Lease shall
expire on ____________.

      If the foregoing is in accordance with your understanding, kindly execute
the enclosed duplicate of this letter, and return the same to us.

                                        Very truly yours,

                                        [Landlord]

                                        By: __________________________________

                                        Name _________________________________

                                        Title: _______________________________

                                                                         Page 21
<PAGE>

                                    EXHIBIT E

                     LESSOR'S IMPROVEMENT TO HERITAGE PLACE

Property improvements to be completed by the commencement of Year 4 are the
following:

      1.    Replacement of all exterior windows in building 2.

      2.    Demolition of Building 4.

      3.    Repave designated Parking area

      4.    Creation of additional parking areas in the rear of the building and
            where the current reservoir is located subject to all necessary
            permits and approvals. This provision is intended to increase total
            available spaces for Lessee's use from 78 to 129 (or as otherwise
            required pursuant to P 35 of the addendum attached hereto.)

                                                                         Page 22
<PAGE>

                                    EXHIBIT F

                        LESSEE'S MATERIAL SUBSTANCE LIST

1.    CYCLOHEXANONE

2.    GLYCEROL

3.    BLEACH

4.    BOVINE BLOOD

5.    HUMAN BLOOD

6.    ALCHOHOL

                                                                         Page 23
<PAGE>

                                    EXHIBIT G

                         LIST OF ENVIRONMENTAL ENGINEERS

<TABLE>
<CAPTION>
                  NAME                              ADDRESS                    PHONE NUMBER
                  ----                     -----------------------       ------------------------
<S>                                        <C>                           <C>
Bois Consulting                            Framingham, MA                508-788-9988

Clean Harbors Environmental Services Inc.  530 East 1st Street           617-269-5830
                                           Boston, MA 02127

ENSR                                       35 Nagog Park                 978-635-9500
                                           Acton, MA 01720

Haley & Aldrich                            465 Medford St. 2200          617-886-7491 direct line
                                           Charlestown, MA 02129         617-886-7400 main line

John Cox                                                                 978-772-5374

Laidlaw Environmental Services Inc.        221 Sutton Street             978-683-1002
                                           North Andover, MA 01845

Occuhealth                                 44 Wood Ave                   508-339-9119
                                           Mansfield, MA 02048

Tighe & Bond                               53 Southampton Rd. 3          413-562-1600
                                           Westfield, MA 01805
</TABLE>

                                                                         Page 24
<PAGE>

                 MODIFICATION TO STANDARD FORM COMMERCIAL LEASE

This Modification is entered into by and between Heritage Place, LLC ("LESSOR")
and NxSTAGE Medical ("LESSEE") with respect to that Standard Form Commercial
Lease dated October 17, 2000 ("LEASE') concerning the property located in
Building Two, 439 South Union Street, Lawrence, MA 01843.

WHEREAS, the leased premises included 1,000 square feet of space in the basement
of the building for the purpose of storage, ("Storage Space")

WHEREAS, the LESSEE has determined that it no longer requires the Storage Space
and desires to surrender it, and to be released from and to terminate LESSEE's
rights and obligations arising under the Lease for the Storage Space, including
LESSEE's use and occupancy of the Storage Space, and

WHEREAS, in recognition thereof, Lessor is willing to accept LESSEE's surrender
of the Storage Space, and the parties have agreed to modify and amend the LEASE,
subject to the terms and conditions set forth herein.

NOW, THEREFORE, in consideration of the mutual covenants and conditions
contained herein, the parties agree as follows:

1.    The parties agree that the Commencement Date, as defined in P 47 of the
      Lease is February 26, 2001

2.    The Term of the Lease for the Storage Space shall expire as of April 30,
      2001 (The "Effective Date"). LESSEE shall vacate and surrender to LESSOR
      the Storage Space on or before the Effective Date.

3.    Provided that LESSEE has timely complied with P 2 hereof, then the rent
      will be reduced at a fixed rate of $3000.00 per year or $250.00 monthly
      for the balance of the Lease Term for the remaining premises, effective
      with the rent payment due for May 2001. The new rent figures are amended
      as follows:

<TABLE>
<CAPTION>
             COST/SF     MONTHLY RENT     ANNUAL RENT
           ----------    ------------     -----------
<S>        <C>           <C>              <C>
YEAR 1     $     9.75    $     34,978     $   420,238
YEAR 2     $    10.25    $     36,772     $   441,263
YEAR 3     $    10.75    $     38,566     $   462,788
YEAR 4     $    11.25    $     40,359     $   484,313
YEAR 5     $    11.75    $     42,153     $   505,838
</TABLE>

      The Monthly rent figure for Year 1, $35,020.00 is applicable to May 2001
      and the remainder of Year 1 only. The Annual rent figure for Year 1
      reflects an additional $500 for rent paid for the Storage Space during
      March and April 2001.

                                                                         Page 25
<PAGE>

4.    The rent increases stipulated above will become effective on the
      respective anniversary of the Commencement Date of the term of the Lease
      set forth in P 1 above.

5.    This Agreement contains the entire agreement by and between the parties
      with respect to the modification of LEASE and may not be amended,
      terminated or otherwise modified without the express written consent of
      all parties hereto.

6.    All other terms and provisions as set forth in the LEASE are hereby
      ratified and reaffirmed.

7.    This Modification is entered into this ____________ day of____________ ,
      2001.

LESSEE:                                        LESSOR:

/s/ Jeffrey H. Burbank                         /s/ Orit Goldstein
-----------------------                        ------------------------
Jeff Burbank, President                        Orit Goldstein, Member
NxSTAGE Medical                                Heritage Place, LLC

                                                                         Page 26

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