Document:

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                                                                   Exhibit 10.20

                              NXSTAGE MEDICAL, INC.

                FIFTH AMENDED AND RESTATED STOCKHOLDERS AGREEMENT

      THIS FIFTH AMENDED AND RESTATED STOCKHOLDERS AGREEMENT (this "Agreement")
is made and entered into this 8th day of July, 2005, by and among NxSTAGE
MEDICAL, INC., a Delaware corporation (the "Company"), Dr. C. David Finch, Dr.
Hendrik K. Kuiper, and Jeffrey H. Burbank (collectively, the "Founders"), A.D. &
S.R. Burbank Trust and J.B. Babcock (together with the Founders, the "Key
Stockholders"), the persons and entities listed on Exhibit B hereto (the "
Initial Investors") and those persons and entities who shall, after the date
hereof, agree to become a party to and be bound by this Agreement by executing
and delivering to the Company a counterpart signature page hereto in such form
as the Company may designate (each an "Additional Investor", and together with
the Initial Investors, the "Investors"). Exhibit B to this Agreement shall be
amended and restated by the Company to reflect the admission of an Additional
Investor.

                                   WITNESSETH:

      WHEREAS, each of the Key Stockholders is the beneficial owner of those
shares of the Company's common stock, par value $0.001 per share (the "Common
Stock") set forth opposite his name on Exhibit A;

      WHEREAS, certain of the Investors (the "Series B Investors," "Series C
Investors", "Series D Investors", "Series E Investors" and "Series F Investors")
hold that number of shares of the Company's Series B, Series C, Series D, Series
E and Series F Preferred Stock, par value $.001 per share (the "Series B, Series
C, Series D Preferred Stock, Series E and Series F Preferred Stock"), set forth
opposite their names on Exhibit B hereto and possess voting rights, rights of
first refusal, co-sale rights and other rights pursuant to that certain Fourth
Amended and Restated Stockholders Agreement dated as of August 18, 2004 by and
among the Company, the Series B Investors, the Series C Investors, the Series D
Investors, the Series E Investors, the Series F Investors and the Key
Stockholders (the "Prior Agreement");

      WHEREAS, the Company proposes to sell to certain of the Investors (the
"Series F-1 Investors") that number of shares of its Series F-1 Preferred Stock,
par value $.001 per share (the "Series F-1 Preferred Stock"), as set forth
opposite such Investor's name on Exhibit B attached hereto pursuant to the terms
of the Series F-1 Preferred Stock Purchase Agreement by and among the Company
and the Series F-1 Investors of even date herewith (the "Purchase Agreement");

      WHEREAS, the Series B, Series C, Series D, the Series E and Series F
Investors are the holders of at least a majority in interest of the Series B,
Series C, Series D, Series E and Series F Preferred Stock held by the Series B,
Series C, Series D, Series E and Series F Investors as of the date hereof and
the Key Stockholders are the holders of at least a majority in interest of the
shares held by Key Stockholders as of the date hereof, and the Series B, Series
C, Series D,
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Series E and Series F Investors and Key Stockholders desire to terminate the
Prior Agreement and to accept the rights created pursuant hereto in lieu of the
rights granted to them under the Prior Agreement; and

      WHEREAS, it is a condition to the closing of the sale of the Series F-1
Preferred Stock pursuant to the Purchase Agreement that this Agreement be
executed and delivered by the Investors, the Key Stockholders and the Company;

      NOW, THEREFORE, in consideration of the mutual promises and covenants set
forth herein, the Series B, Series C, Series D, Series E and Series F Investors,
the Key Stockholders and the Company hereby agree that the Prior Agreement shall
be superseded and replaced in its entirety by this Agreement, and the parties
hereto further agree as follows:

                                   ARTICLE I

                                     VOTING

      1.1 COMMON SHARES; INVESTOR SHARES.

            (A) The Key Stockholders each agree to hold all shares of voting
capital stock of the Company registered in their respective names or
beneficially owned by them as of the date hereof, and any and all other
securities of the Company legally or beneficially acquired by each of the Key
Stockholders after the date hereof (hereinafter collectively referred to as the
"Common Shares") subject to, and to vote the Common Shares in accordance with,
the provisions of this Agreement. Common Shares shall not include shares of
Common Stock issued upon conversion of shares of Series B Preferred Stock,
Series C Preferred Stock, Series D Preferred Stock, Series E Preferred Stock,
Series F Preferred Stock or Series F-1 Preferred Stock.

            (B) The Investors each agree to hold all shares of voting capital
stock of the Company now owned or hereinafter acquired by them (including, but
not limited to, all shares of Common Stock issued upon conversion of the Series
B Preferred Stock, Series C Preferred Stock, Series D Preferred Stock, Series E
Preferred Stock, Series F Preferred Stock and Series F-1 Preferred Stock (the
"Conversion Shares"), as adjusted for any combinations, consolidations,
recapitalizations, stock splits, reverse stock splits, stock distributions or
stock dividends with respect to such shares) registered in their respective
names or beneficially owned by them as of the date hereof (and any and all other
securities of the Company legally or beneficially acquired by each of the
Investors after the date hereof) (hereinafter collectively referred to as the
"Investor Shares") subject to, and to vote the Investor Shares in accordance
with, the provisions of this Agreement.

      1.2 VOTING. At each election of directors in which the holders of Common
Stock, holders of Series B Preferred Stock, holders of Series C Preferred Stock,
holders of Series D Preferred Stock, holders of Series E Preferred Stock,
holders of Series F Preferred Stock and holders of Series F-1 Preferred Stock,
voting together as a single class, are entitled to elect directors of the
Company, the Key Stockholders and Investors shall consult each other and shall
vote Common Shares and Investor Shares, respectively so that at least (i) two
directors shall be

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nominees of the holders of a majority in interest of the Common Stock, voting as
a separate class, one of whom it is anticipated will be the Chief Executive
Officer of the Company, and one of whom shall be an industry expert and an
outside director who shall be reasonably satisfactory to the holders of a
majority in interest of the Series B Preferred Stock, Series C Preferred Stock,
Series D Preferred Stock, Series E Preferred Stock, Series F Preferred Stock and
Series F-1 Preferred Stock, and (ii) five directors shall be nominees of the
holders of a majority in interest of the Series B Preferred Stock, Series C
Preferred Stock, Series D Preferred Stock, Series E Preferred Stock, Series F
Preferred Stock and Series F-1 Preferred Stock, voting together as a single
class, on an as-if-converted basis and not as separate series, (A) one of whom
shall be an industry expert and outside director which shall be reasonably
satisfactory to the holders of a majority in interest of the Common Stock and
BVCF IV, L.P., (B) one of which shall be designated by David S. Utterberg (or
upon his death, his estate) as long as Mr. Utterberg's ownership in NxStage does
not fall below 820,019 shares (as adjusted for any combinations, consolidations,
recapitalizations, stock splits, reverse stock splits, stock distributions or
stock dividends with respect to such shares) ("DSU Minimum Shares"), (C) one of
which shall be designated by the Sprout Group as long as the Sprout Group's
(together with its affiliates) ownership in NxStage does not fall below DSU
Minimum Shares, (D) one of which shall be designated by Atlas Venture Fund V,
L.P. as long as Atlas Venture Fund V, L.P.'s (together with its affiliates)
ownership in NxStage does not fall below DSU Minimum Shares and (E) one of which
shall be designated by Healthcare Investment Partners Holdings LLC as long as
Healthcare Investment Partners Holdings LLC's (together with its affiliates)
ownership in NxStage does not fall below DSU Minimum Shares. 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.

      1.3 LEGEND.

            (A) Concurrently with the execution of this Agreement, there shall
be imprinted or otherwise placed, on certificates representing the Common Shares
and the Investor Shares a restrictive legend in substantially the following form
(the "Legend"):

            "THE SHARES REPRESENTED BY THIS CERTIFICATE ARE SUBJECT TO THE TERMS
            AND CONDITIONS OF A FIFTH AMENDED AND RESTATED STOCKHOLDERS
            AGREEMENT DATED AS OF JULY 8, 2005, WHICH PLACES CERTAIN
            RESTRICTIONS ON THE VOTING OF THE SHARES REPRESENTED HEREBY. ANY
            PERSON ACCEPTING ANY INTEREST IN SUCH SHARES SHALL BE DEEMED TO
            AGREE TO AND SHALL BECOME BOUND BY ALL THE PROVISIONS OF SUCH
            AGREEMENT. A COPY OF SUCH FIFTH AMENDED AND RESTATED STOCKHOLDERS
            AGREEMENT WILL BE FURNISHED TO THE RECORD HOLDER OF THIS CERTIFICATE
            WITHOUT CHARGE UPON WRITTEN REQUEST

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            TO THE CORPORATION AT ITS PRINCIPAL PLACE OF BUSINESS."

            (B) The Company agrees that, during the term of this Agreement, it
will not remove, and will not permit to be removed (upon registration of
transfer, reissuance of otherwise), the Legend from any such certificate and
will place or cause to be placed the Legend on any new certificate issued to
represent the Common Shares or the Investor Shares theretofore represented by a
certificate carrying the Legend.

      1.4 SUCCESSORS. The provisions of this Agreement shall be binding upon the
successors in interest to any of the Common Shares or Investor Shares. The
Company shall not permit the transfer of any of the Common Shares or Investor
Shares on its books or issue a new certificate representing any of the Common
Shares or Investor Shares unless and until the person to whom such security is
to be transferred shall have executed a written agreement, pursuant to which
such person becomes a party to this Agreement and agrees to be bound by all the
provisions hereof as if such person were a Key Stockholder or Investor, as
applicable.

      1.5 OTHER RIGHTS. Except as provided by this Agreement, each Key
Stockholder and Investor shall exercise the full rights of a shareholder with
respect to the Common Shares and the Investor Shares, respectively.

      1.6 BOARD OBSERVATION RIGHTS. So long as (i) BVCF IV, L.P. shall continue
to hold at least 500,000 shares of the Company's Preferred Stock (as adjusted
for any combinations, consolidations, recapitalizations, stock splits, reverse
stock splits, stock distributions or stock dividends with respect to such
shares), (ii) Marubeni Corporation shall continue to hold at least 300,000
shares of the Company's Preferred Stock (as adjusted for any combinations,
consolidations, recapitalizations, stock splits, reverse stock splits, stock
distributions or stock dividends with respect to such shares) and (iii) CSFB
Fund Co-Investment Program, L.P. shall continue to hold at least 200,000 shares
of the Company's Preferred Stock (as adjusted for any combinations,
consolidations, recapitalizations, stock splits, reverse stock splits, stock
distributions or stock dividends with respect to such shares), a representative
of each, reasonably acceptable to the Company (a "Representative"), shall have
the right to attend all meetings of the Company's Board of Directors in a
non-voting observer capacity and, in this respect, the Company shall give the
Representative, whether or not present at such meetings, copies of all notices,
minutes, consents and other materials that it provides to its directors in the
same manner and at the same time as provided to its directors; provided that (A)
the Representative shall agree to hold in strict confidence and trust, and to
act in a fiduciary manner with respect to, all information so provided; (B) the
Company reserves the right to withhold any information and to exclude the
Representative from any portion of any meeting, if the Board of Directors
determines in good faith that access to such information or attendance at such
portion of such meeting could materially and adversely affect the Company,
whether by way of adversely affecting the attorney-client privilege between the
Company and its counsel, or otherwise, and (C) in no event shall the failure to
provide the notice and materials described above invalidate in any way any
action taken at a meeting of the Company's Board of Directors. Such
Representatives shall bear their own costs associated with such attendance.

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                                   ARTICLE II

                              TRANSFER RESTRICTIONS

      2.1 COMPANY PURCHASE RIGHT; INVESTOR PURCHASE RIGHT. If a Founder receives
a bona fide offer to purchase any of the Common Shares held by such Founder, the
Company and its assignees shall have a right of first refusal to purchase from
Founder all, but not less than all, of the Common Shares that such Founder
proposes to transfer, assign or otherwise dispose of on the same terms and
conditions as those contained in the Offer Notice (defined below) (the "Company
Purchase Right"). If the Company should decide not to exercise such Company
Purchase Right with regard to any such Common Shares purchasable thereunder,
each Investor shall then have the right of first refusal to purchase his, her or
its pro rata share of such Common Shares on the same terms and conditions as
those contained in the Offer Notice (the "Investor Purchase Right"). Such
Investor's pro rata share, for purposes of the Investor Purchase Right, is the
ratio, the numerator of which is the number of shares of Common Stock issued or
issuable to the Investor pursuant to the conversion of all shares of Series B
Preferred Stock, Series C Preferred Stock, Series D Preferred Stock, Series E
Preferred Stock, Series F Preferred Stock and Series F-1 Preferred Stock held by
the Investor, and the denominator of which is the total number of shares of
Common Stock issued or issuable upon conversion of all outstanding shares of
Series B Preferred Stock, Series C Preferred Stock, Series D Preferred Stock,
Series E Preferred Stock, Series F Preferred Stock and Series F-1 Preferred
Stock. The Company Purchase Right and the Investor Purchase Right shall be
subject to the following provisions:

            (A) "Offered Stock" shall mean any Common Shares that the Company
has the opportunity to purchase under the Company Purchase Right.

            (B) At least twenty (20) days prior to any transfer of Common Shares
by any of the Founders (other than a transfer to a Permitted Transferee (as
defined below)), the Founder proposing to transfer such Common Shares (the
"Transferor"), shall deliver to the Company and to each of the Investors a
written notice (the "Offer Notice") specifying in reasonable detail the number
of Common Shares to be transferred and the price and terms upon which such
Common Shares are to be transferred. Within twenty (20) days from receipt of the
Offer Notice (the "Company Option Period"), the Company must send a written
notice to the Transferor and each of the Investors of its intention to purchase
the Offered Stock (the "Company Election Notice"). If the Company does not
intend to purchase all of the Offered Stock or the Company is not lawfully able
to purchase all such Offered Stock, the Company Election Notice shall state the
amount of Offered Stock that the Company will not purchase, and shall specify
each Investor's pro rata share thereof. The Investors shall have the right to
purchase all, but not less than all, of the Offered Stock not purchased by the
Company.

            (C) Each Investor desiring to purchase such Offered Stock shall have
ten (10) days (the "Investor Option Period") from the date of mailing of the
Company Election Notice to provide written notice to the Transferor and the
Company of the number of shares of such Offered Stock that such Investor desires
to purchase (such number may be more than such Investor's pro rata share) (the
"Investor Election Notice"). None of the Investors shall have the right to
purchase any of such Offered Stock unless the Investors in the aggregate agree
to

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purchase all of the Offered Stock not purchased by the Company or by other
Company shareholders.

            (D) If the total number of shares specified in Investor Election
Notices exceeds the number of shares of Offered Stock not purchased by the
Company on a pro rata basis no more favorable than the pro rata rights of the
Investors, then (unless the exercising Investors agree otherwise) each
exercising Investor shall have the right to purchase that number of shares of
Offered Stock that is obtained by multiplying the number of shares of Offered
Stock not being purchased by the Company or other shareholders as described
above by a fraction the numerator of which is the number of shares of Common
Stock issued or issuable to such exercising Investor pursuant to the conversion
of all shares of Preferred Stock held by such Investor, and the denominator of
which is the total number of shares of Common Stock issued or issuable to all
such exercising Investors pursuant to the conversion of all shares of Preferred
Stock held by all such exercising Investors. Any remaining shares of Offered
Stock may be purchased by exercising Investors that so elect, according to the
same principle of proration until all shares of Offered Stock not purchased by
the Company or other shareholders as described above are allocated to exercising
Investors.

            (E) Within five (5) days after the expiration of the Investor Option
Period the Company shall give written notice (the "Expiration Notice") to the
Transferor and to each Investor specifying either (i) that all of the Offered
Stock was subscribed by the Company exercising its Company Purchase Right and/or
one or more Investors exercising their Investor Purchase Right or (ii) that
neither the Company nor the Investors have the right to purchase any of the
Offered Stock because the Investors, in the aggregate, did not timely exercise
their Investor Purchase Right to purchase all of the Offered Stock not purchased
by the Company.

            (F) This Investor Purchase Right is assignable to any affiliate of
an Investor or in connection with a sale of Investor Shares, provided that the
transferee owns, at the time the Offered Stock is offered, at least two hundred
thousand (200,000) shares of Series B Preferred Stock, Series C Preferred Stock,
Series D Preferred Stock, Series E Preferred Stock, Series F Preferred Stock,
Series F-1 Preferred Stock or Conversion Shares (as adjusted for any
combinations, consolidations, recapitalizations, stock splits, reverse stock
splits, stock distributions or stock dividends with respect to such shares).

      2.2 RIGHTS OF CO-SALE.

            (A) Investors Co-Sale Option Covering Founder Shares. In the event
any Founder (including for all purposes of this Section 2.2 any Permitted
Transferees of a Founder) proposes to sell any Common Shares and any of such
shares are not purchased pursuant to Section 2.1 (the "Remaining Shares"), such
Founder may transfer the Remaining Shares only following compliance with this
Section 2.2(a):

                  (i) In such event, immediately upon receipt of the Expiration
Notice, the Transferor shall give an additional notice of the proposed sale to
the Investors if applicable, which shall identify the offeror and the number of
Remaining Shares proposed to be sold (the "Co-Sale Notice").

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                  (ii) Each of the Investors shall have the right, exercisable
upon written notice to the Transferor or any Permitted Transferee within 10 days
after the date of the Co-Sale Notice (the "Co-Sale Notice Period"), to
participate in the sale on the terms and conditions stated in the Co-Sale
Notice. Each of the Investors shall have the right to sell all or any portion of
its Investor Shares on the terms and conditions in the Co-Sale Notice (subject
to the foregoing), with the maximum number of Investor Shares equal to the
product obtained by multiplying the number of Remaining Shares proposed to be
sold as described in the Co-Sale Notice by a fraction, the numerator of which is
the number of Investor Shares owned by such Investor on the date of the Co-Sale
Notice, and the denominator of which is the sum of the number of shares of
Common Stock owned by the Founders and their Permitted Transferees and the
number of Investor Shares owned by all of the Investors as of the date of the
Co-Sale Notice. To the extent an Investor elects not to sell the full amount of
Investor Shares which they are entitled to sell pursuant to this Section 2.2,
the other participating Investors' rights to sell shares shall be increased
proportionately to their relative holdings of Investor Shares, such that the
Investors shall have the right to sell the full number of shares allocable to
them in the aggregate in any transaction subject to this Section 2.2 even if
some Investors elect not to participate.

                  (iii) Within five (5) days after the expiration of the Co-Sale
Notice Period, the Transferor shall notify each participating Investor of the
number of shares held by such Investor that will be included in the sale and the
date on which the sale will be consummated.

                  (iv) Each of the Investors may effect its participation in any
sale hereunder by delivery to the purchaser, or to the Transferor for transfer
to the purchaser, of one or more instruments, certificates and/or option
agreements, properly endorsed for transfer, representing the shares it elects to
sell therein, provided that no Investor shall be required to make any
representations or warranties or to provide any indemnities in connection
therewith other than with respect to title to the stock being conveyed. At the
time of consummation of the sale, the purchaser shall remit directly to each
Investor that portion of the sale proceeds to which such Investor is entitled by
reason of its participation therein. No shares may be purchased by a purchaser
from the Transferor or Permitted Transferee unless the purchaser simultaneously
purchases from the Investors all of the shares that they have elected to sell
pursuant to this Section 2.2(a).

            (B) Investors/Founders Co-Sale Option covering Investor Shares. In
the event that any Investor (a "Transferring Investor") receives a bona fide
offer to purchase (an "Investor Offer") all or any portion of the Investor
Shares (the "Investor Offered Shares") from an offeror other than another
Investor or an affiliate of such Transferring Investor, such Transferring
Investor may transfer the Investor Offered Shares only pursuant to and in
accordance with the following provisions of this Section 2.2(b):

                  (i) Such Transferring Investor shall deliver a written notice
containing equivalent information to that required under Section 2.1 hereof to
the Company and each of the Investors and Founders (an "Investor Offer Notice").
Each of the Investors and Founders shall have the right to participate in the
Investor Offer on the terms and conditions herein stated (the "Investor/Founder
Co-Sale Option"), which right shall be exercisable upon written notice (an
"Acceptance Notice") to the Transferring Investor within the 10 days after the
date of the

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Investor Offer Notice (the "Investor/Founder Co-Sale Notice Period"). The
Acceptance Notice shall indicate the maximum number of Investor Shares such
Investor or Founder wishes to sell (including the number of shares it would sell
if one or more other Investors or Founders do not elect to participate in the
sale) on the terms and conditions stated in the Investor Offer Notice.

                  (ii) Each of the Investors and Founders shall have the right
to sell a portion of such Investor's Investor Shares or Founder's Common Shares,
as the case may be, pursuant to the Investor Offer Notice which is equal to or
less than the product obtained by multiplying the number of shares proposed to
be sold in the Investor Offer Notice by a fraction, the numerator of which is
the number of Investor Shares or Common Shares owned by such Investor or
Founder, as the case may be, on the date of the Investor Offer Notice, and the
denominator of which is the sum of the number of shares of Common Shares owned
by the Founders and their Permitted Transferees and the number of shares of
Investor Shares owned by all of the Investors as of the date of the Investor
Offer Notice. To the extent an Investor or Founder elects not to sell the full
amount of Investor Shares or Common Shares which they are entitled to sell
pursuant to this Section 2.2(b), the other participating Investors' and
Founders' rights to sell shares shall be increased proportionately by the full
amount of Investor Shares or Common Shares which the non-electing Investors or
Founders were entitled to sell pursuant to this Section 2.2(b).

                  (iii) Within five (5) days after the expiration of the
Investor/Founder Co-Sale Notice Period, the Transferring Investor shall notify
each participating Investor and Founder of the number of shares held by such
Investor or Founder that will be included in the sale and the date on which the
Investor Offer will be consummated.

                  (iv) Each of the Investors and Founders participating in the
sale pursuant to this Section 2.2(b) may effect its participation in any
Investor Offer hereunder by delivery to the purchaser, or to the Transferring
Investor for delivery to the purchaser, of one or more instruments or
certificates, properly endorsed for transfer, representing the Common Shares or
Investor Shares it elects to sell therein, provided that no Investor or Founder
shall be required to make any representations or warranties or provide any
indemnities in connection therewith other than with respect to title to the
stock being conveyed. At the time of consummation of the sale, the purchaser
shall remit directly to each Investor or Founder that portion of the sale
proceeds to which such Investor or Founder is entitled by reason of its
participation therein. No Investor Offered Shares may be purchased by a
purchaser from the relevant Investor or any of such Investor's Permitted
Transferees unless the purchaser simultaneously purchases from the other
Investors or Founders all of the shares that they have elected to sell pursuant
to this Section 2.2(b).

      2.3 SALE PERIOD. Any shares held by an Investor or Founder or any of their
Permitted Transferees that the Investor, Founder or transferee desires to sell
following compliance with this Section 2.3 may be sold to a purchaser:

                  (i) by the later of (A) the 90-day period after the expiration
of the Co-Sale Notice Period or Investor/Founder Co-Sale Notice Period, as
applicable, or (B) the date upon which all governmental approval requirements,
if any, have been satisfied (the "Sale Period"); and

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                  (ii) only on terms no more favorable than those contained in
the relevant notice to the Investor or Founder.

      Promptly after such sale, such Investor, Founder or Permitted Transferee
shall notify the parties hereto of the consummation thereof and shall furnish
such evidence of the completion and time of completion of such sale and of the
terms thereof as may reasonably be requested by the other parties hereto. If, at
the end of such Sale Period, such Investor, Founder or any Permitted Transferees
have not completed the sale of such shares as aforesaid, all the restrictions on
the transfer of shares contained in this Section 2 shall again be in effect with
respect to such shares.

      2.4 PERMITTED TRANSFERS.

            (A) Subject to Section 2.4(b), but notwithstanding any other
provision of this Agreement, the provisions of Sections 2.1 and 2.2 shall not
apply to:

                  (i) Any transfer of Common Shares or Investor Shares, as
applicable, by a Founder or Investor that is an individual to such Founder's or
Investor's Family Group (for purposes of this Agreement, "Family Group" means
the Founder's or Investor's spouse, descendants (whether natural or adopted),
parents, siblings and any trust, partnership, limited liability company or
similar entity created solely for the benefit of such Founder or Investor and/or
any such Founder's or Investor's spouse, descendants, parents and/or siblings);
or

                  (ii) Any transfer by a Founder or Investor that is not an
individual to such Founder's or Investor's members, former members, partners,
former partners, stockholders or affiliates.

      (B) In the event of any transfer pursuant to Subsection 2.4(a), the
transferee (a "Permitted Transferee") of any such shares shall hold the shares
so acquired with all the rights conferred by, and subject to all the
restrictions imposed by, this Agreement (including, without limitation, those
set forth in Sections 1.2 and 4.5), and as a condition to any such transfer, the
Permitted Transferee shall execute and deliver to the Company an instrument of
accession in a form acceptable to the Company agreeing to become a party to and
be bound by the provisions of this Agreement to the same extent as if it/he were
an original party hereto in the same capacity as the transferor.

                                  ARTICLE III

                                   TERMINATION

      3.1 This Agreement shall continue in full force and effect from the date
hereof through the earliest of the following dates, on which it shall terminate
in its entirety:

            (A) the date of the closing of a Qualified Public Offering (as
defined in the Company's Certificate of Incorporation, as amended); or

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            (B) at such time as the Investors hold less than an aggregate of one
hundred thousand (100,000) shares of Series B, Series C, Series D, Series E,
Series F and Series F-1 Preferred Stock (as adjusted for stock splits, stock
dividends, recapitalizations and the like); or

            (C) ten (10) years from the date of this Agreement; or

            (D) the date as of which the parties hereto terminate this Agreement
by written consent of a majority in interest of the Investors and a majority in
interest of the Key Stockholders.

                                   ARTICLE IV

                                  MISCELLANEOUS

      4.1 OWNERSHIP. Each Key Stockholder represents and warrants to the
Investors that such Key Stockholder now owns the Common Shares, free and clear
of liens or encumbrances, and has not, prior to or on the date of this
Agreement, executed or delivered any proxy or entered into any other voting
agreement or similar arrangement other than the Prior Agreement or one which has
expired or terminated prior to the date hereof, and such Key Stockholder has
full power and capacity to execute, deliver and perform this Agreement, which
has been duly executed and delivered by, and evidences the valid and binding
obligation of, such Key Stockholder enforceable in accordance with its terms.

      4.2 FURTHER ACTION. If and whenever the Common Shares or the Investor
Shares are sold, the Key Stockholders or the personal representative of the Key
Stockholders and the Investors shall do all things and execute and deliver all
documents and make all transfers, and cause any transferee of the Common Shares
to do all things and execute and deliver all documents, as may be necessary to
consummate such sale consistent with this Agreement.

      4.3 SPECIFIC PERFORMANCE. The parties hereto hereby declare that it is
impossible to measure in money the damages which will accrue to a party hereto
or to their heirs, personal representatives, or assigns by reason of a failure
to perform any of the obligations under this Agreement and agree that the terms
of this Agreement shall be specifically enforceable. If any party hereto or his
heirs, personal representatives, or assigns institutes any action or proceeding
to specifically enforce the provisions hereof, any person against whom such
action or proceeding is brought hereby waives the claim or defense therein that
such party or such personal representative has an adequate remedy at law, and
such person shall not offer in any such action or proceeding the claim or
defense that such remedy at law exists.

      4.4 GOVERNING LAW. This Agreement, and the rights of the parties hereto,
shall be governed by and construed in accordance with the laws of the State of
Delaware as such laws apply to agreements among Delaware residents made and to
be performed entirely within the State of Delaware.

      4.5 AMENDMENT. This Agreement may be amended only by an instrument in
writing signed by the Company, a majority in interest of the Investors and a
majority in interest of the

                                       10
<PAGE>
Key Stockholders; provided that no amendments may be made to (i) Section
1.2(ii)(A) hereof which adversely alter the rights of BVCF IV, L.P. without the
written consent of BVCF IV, L.P. so long as BVCF IV, L.P. maintains its full pro
rata share in the Company measured as of the original issuance date of the
Series D Preferred Stock, or (ii) Section 1.2(ii)(B), (C), (D) or (E) hereof
which adversely alters the rights of David Utterberg, the Sprout Group, Atlas
Venture Fund V, L.P., or Healthcare Investment Partners Holdings LLC,
respectively, without the written consent of Mr. Utterberg, the Sprout Group,
Atlas Venture Fund V, L.P., or Caxton Healthcare Acquisition Partners,
respectively. For purposes of this Section 4.5(i) BVCF IV, L.P.'s "full pro rata
share" shall mean the quotient of the total number of shares of Preferred Stock
held by BVCF IV, L.P. and its affiliates divided by the total number of shares
of Preferred Stock issued by the Company.

      4.6 SEVERABILITY. If any provision of this Agreement is held to be invalid
or unenforceable, the validity and enforceability of the remaining provisions of
this Agreement shall not be affected thereby.

      4.7 SUCCESSORS. This Agreement shall inure to the benefit of and be
binding upon the parties hereto and their respective heirs, successors, assigns,
administrators, executors and other legal representatives.

      4.8 ADDITIONAL SHARES. In the event that subsequent to the date of this
Agreement any shares or other securities (other than any shares or securities of
another corporation issued to the Company's shareholders pursuant to a plan of
merger) are issued on, or in exchange for, any of the Common Shares or Investor
Shares by reason of any stock dividend, stock split, consolidation of shares,
reclassification or consolidation involving the Company, such shares or
securities shall be deemed to be Common Shares or Investor Shares, as the case
may be, for purposes of this Agreement.

      4.9 COUNTERPARTS. This Agreement may be executed in one or more
counterparts, each of which will be deemed an original but all of which together
shall constitute one and the same agreement.

      4.10 WAIVER. No waivers of any breach of this Agreement extended by any
party hereto to any other party shall be construed as a waiver of any rights or
remedies of any other party hereto or with respect to any subsequent breach.

      4.11 ATTORNEY'S FEES. In the event that any suit or action is instituted
to enforce any provision in this Agreement, the prevailing party shall be
entitled to all costs and expenses of maintaining such suit or action, including
reasonable attorneys' fees.

      4.12 NOTICES. Any and all notices or other communications required or
permitted to be given under any of the provisions of this Agreement shall be in
writing and shall be deemed to have been made when given to the address and
telephone numbers set forth opposite such Key Stockholder or Investor's name on
Exhibit A or Exhibit B attached hereto, with a copy to Goodwin | Procter LLP,
Exchange Place, Boston, MA 02109, Attn: Mitchell S. Bloom, Esq., (telephone:
617-570-1055; facsimile: 617-523-1231) or in the case of the Company to:

      NxSTAGE MEDICAL, INC.

                                       11
<PAGE>
      439 South Union Street, 5th Floor
      Lawrence, MA  01843
      Attn:  General Counsel

with a copy (which shall not constitute notice) to:

      Wilmer Cutler Pickering Hale and Dorr, LLP
      60 State Street
      Boston, MA 02109
      Attn: Susan Murley, Esq.

(or at such other address or telefax numbers as a party may specify by notice to
the other party given as aforesaid). Unless otherwise specifically provided in
this Agreement, such communication shall be deemed to have been given (a) three
days after mailing, when mailed by registered or certified postage-paid mail,
(b) on the next business day, when delivered to a same-day or overnight national
courier service or the U.S. Post Express Mail or (c) upon the date of receipt by
the addressee when delivered personally or (d) upon the date of receipt by the
addressee when delivered or by telecopier if such telecopy notice is followed by
any of the methods of delivery set forth in (a) through (c) of this section;
provided, however, that any notice of change of address shall be effective only
upon receipt. Notice may be given on behalf of a party by its counsel.

         [THE REMAINDER OF THIS PAGE HAS BEEN INTENTIONALLY LEFT BLANK]

                                       12
<PAGE>
      IN WITNESS WHEREOF, the parties hereto have executed this Agreement as of
the date first above written.

                                 COMPANY:

                                 NXSTAGE MEDICAL, INC.

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

                                 KEY STOCKHOLDERS:

                                 A.D. & S.R. BURBANK TRUST

                                 By:   /s/ A. David Burbank
                                    --------------------------------------
                                       A. David Burbank, Co-Trustee

                                 By:   /s/ Sally R. Burbank
                                    ----------------------------------------
                                       Sally R. Burbank, Co-Trustee

                                       /s/ Jeffrey H. Burbank
                                 ------------------------------
                                       Jeffrey H. Burbank
<PAGE>
                                       /s/ C. David Finch
                                 ------------------------------------------
                                       C. David Finch

                                       /s/ Hendrik K. Kuiper
                                 ------------------------------------------
                                       Hendrik K. Kuiper

                                       /s/ James B. Babcock
                                 ------------------------------------------
                                       James B. Babcock
<PAGE>
                                 INVESTORS:

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

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

                                       /s/  Paul Brown
                                 ------------------------------------------
                                       Paul Brown
<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
<PAGE>
                                 BVCF IV, L.P.

                                 By:   Adams Street Partners, LLC
                                       Its General Partner

                                 By:   /s/ Craig S. Taylor
                                    ---------------------------------
                                       Name: Craig S. Taylor
                                       Title: Partner
<PAGE>
                                 WPG ENTERPRISE FUND III, 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, 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
<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
<PAGE>
                                 SPROUT CAPITAL VIII, L.P.

                                 By:   DLJ Capital Corporation
                                       Its Managing General Partner

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

                                 SPROUT VENTURE CAPITAL, L.P.

                                 By:   DLJ Capital Corporation
                                       Its General Partner

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

                                 DLJ CAPITAL CORP.

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

                                 DLJ ESC II, L.P.

                                 By:   DLJ LBO Plans Management Corporation
                                       Its General Partner

                                 By:   /s/ Philippe Chambon
                                    ---------------------------------
                                       By:  Philippe Chambon
                                       Its:  Attorney In Fact
<PAGE>
                                 SPROUT ENTREPRENEURS FUND, L.P.

                                 By:   DLJ Capital Corp.
                                       Its General Partner

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

                                 SPROUT CAPITAL IX, L.P.

                                 By:   DLJ Capital Corp.
                                       Its Managing General Partner

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

                                 SPROUT IX PLAN INVESTORS, L.P.

                                 By:   DLJ LBO Plans Management Corporation II
                                       Its General Partner

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

                                 THE SPROUT CEO FUND, L.P.

                                 By:   DLJ Capital Corporation
                                       Its General Partner

                                 By:   /s/ Philippe Chambon
                                    ---------------------------------
                                       By:  Philippe Chambon
                                       Its: Managing Director
<PAGE>
                                 SPROUT CAPITAL VII, L.P.

                                 By:   DLJ Capital Corporation
                                       Its Managing General Partner

                                 By:   /s/ Philippe Chambon
                                    ---------------------------------
                                       By: Philippe Chambon
                                       Its: Managing Director
<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
<PAGE>
                                 WASATCH FUNDS, INC. FOR WASATCH ULTRA GROWTH
                                 FUND

                                 By:    /s/ Venice F. Edwards
                                     ------------------------
                                       Name:  Venice F. Edwards
                                       Title: Secretary
<PAGE>
                                    EXHIBIT A

                            LIST OF KEY STOCKHOLDERS

<TABLE>
<CAPTION>
                                              OPTION FOR          TOTAL
                          NUMBER OF SHARES    THE PURCHASE        COMMON STOCK
NAME AND ADDRESS          OF COMMON STOCK     OF COMMON 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. Babcock                     2,030                -0-             2,030
10626-G York Road
Hunt Valley, MD 21030
(410) 666-8500
(410) 666-6267 (Fax)
</TABLE>
<PAGE>
                                    EXHIBIT B

                                LIST OF INVESTORS

<TABLE>
<CAPTION>
                                   Number of       Number of        Number of      Number of      Number of        Number of
                                     Shares         Shares            Shares        Shares          Shares         Shares of
                                   of Series B    of Series C      of Series D    of Series E    of 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
One North Wacker Drive
Suite 2200
Chicago, IL  60606-2807

WPG Enterprise Fund III, L.L.C.      132,054         84,344            65,895        10,983          9,006             3,467
Weiss, Peck & Greer, LLC
555 California Street,
Suite 3130                                                                                                        CLOSING DATE:
San Francisco,                                                                                                     2,866 Shares
California 94104                                                                                                    on 7/08/05
Fax: (415) 989-5108
                                                                                                                    601 Shares
                                                                                                                    on 07/15/05

Weiss, Peck & Greer Venture
Associates IV, L.L.C.                 151,057         96,481            75,377        12,563          10,302            3,966
Weiss, Peck & Greer, LLC
555 California Street,
Suite 3130                                                                                                         CLOSING DATE:
San Francisco, CA 94104                                                                                            3,279 Shares
Fax: (415) 989-5108                                                                                                 on 7/08/05

                                                                                                                     687 Shares
                                                                                                                     on 07/15/05

Weiss, Peck & Greer Venture
Associates IV                          19,003         12,137            9,482          1,580          1,296              498
Cayman, L.P.
c/o Weiss, Peck & Greer, LLC                                                                                       CLOSING DATE:
555 California Street                                                                                               412 Shares
Suite 3130                                                                                                          on 7/08/05
San Francisco, CA 94104
Fax: (415) 989-5708                                                                                                  86 Shares
                                                                                                                     on 07/15/05

</TABLE>
<PAGE>
<TABLE>
<S>                                    <C>          <C>            <C>             <C>            <C>                   <C>
David S. Utterberg                     201,410      128,642         753,769        753,769        343,195               109,220
c/o Medisystems Corporation
701 Pike Street -16th Floor,
Seattle, WA 98101-3016
Copy to John Willett, Esq
Arnold & Porter
399 Park Avenue
New York, NY  10022-4690

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 III, L.P.                          3,572        2,281           1,577          4,192          2,576                  820
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'
Fund V, L.P.                             -0-          -0-            18,860         7,060          4,339                 1,381
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
</TABLE>
<PAGE>
<TABLE>
<S>                                     <C>         <C>            <C>             <C>            <C>                  <C>
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

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-
</TABLE>
<PAGE>
<TABLE>
<S>                                   <C>          <C>             <C>            <C>            <C>               <C>
4-2, Ohtemachi 1-chome
Chiyoda-ku
Tokyo, Japan

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

Wasatch Ultra Growth Fund                -0-          -0-             -0-            -0-          206,044                 -0-
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-                 153,485
c/o Wasatch Small Cap
Growth Fund
150 Social Hall Avenue,
4th Floor
Suite 400                                                                                                           CLOSING DATE:
Salt Lake City, Utah 84111                                                                                         65,572 Shares
                                                                                                                      on 7/08/05

                                                                                                                    87,913 Shares
                                                                                                                     on 07/15/05

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

Healthcare Investment Partners
Holdings LLC                             -0-          -0-             -0-            -0-            -0-               1,373,626
4900 West Dry Creek Rd.
Healdsburg, CA  95448

The Bander Family
Partnership, LP                          -0-          -0-             -0-            -0-            -0-                13,736
12909 Des Peres Woods Drive
St. Louis, MO  63131

Michael J. Carbon                        -0-          -0-             -0-            -0-            -0-                34,340
14 Olympia Court
Oak Brook, IL  60523                                                                                                CLOSING DATE:
                                                                                                                      07/15/05
                                      ---------    ---------       ---------      ---------      ---------            ---------
TOTAL                                 1,875,000    1,151,632       4,857,622      2,669,908      2,747,253            2,197,801
</TABLE><PAGE>

                                                                   EXHIBIT 10.21

                            INDEMNIFICATION AGREEMENT

      This Agreement is made as of the _____ day of _______ 2005, by and between
NxStage Medical, Inc., a Delaware corporation (the "Corporation), and
_______________ (the "Indemnitee"), a director or officer of the Corporation.

      WHEREAS, it is essential to the Corporation to retain and attract as
directors and officers the most capable persons available, and

      WHEREAS, the substantial increase in corporate litigation subjects
directors and officers to expensive litigation risks at the same time that the
availability of directors' and officers' liability insurance has been severely
limited, and

      WHEREAS, it is now and has always been the express policy of the
Corporation to indemnify its directors and officers, and

      WHEREAS, the Indemnitee does not regard the protection available under the
Corporation's Certificate of Incorporation and insurance as adequate in the
present circumstances, and may not be willing to serve or continue to serve as a
director or officer without adequate protection, and

      WHEREAS, the Corporation desires the Indemnitee to serve, or continue to
serve, as a director or officer of the Corporation.

      NOW THEREFORE, the Corporation and the Indemnitee do hereby agree as
follows:

      1. Agreement to Serve. The Indemnitee agrees to serve or continue to serve
as a director or officer of the Corporation for so long as the Indemnitee is
duly elected or appointed or until such time as the Indemnitee tenders a
resignation in writing.

      2. Definitions. As used in this Agreement:

            (a) The term "Proceeding" shall include any threatened, pending or
completed action, suit, arbitration, alternative dispute resolution proceeding,
administrative hearing or other proceeding, whether brought by or in the right
of the Corporation or otherwise and whether of a civil, criminal, administrative
or investigative nature, and any appeal therefrom.

            (b) The term "Corporate Status" shall mean the status of a person
who is or was a director or officer of the Corporation, or is or was serving, or
has agreed to serve, at the request of the Corporation, as a director, officer,
partner, trustee, member, employee or agent of another corporation, partnership,
joint venture, trust, limited liability company or other enterprise.

            (c) The term "Expenses" shall include, without limitation,
attorneys' fees, retainers, court costs, transcript costs, fees and expenses of
experts, travel expenses, duplicating costs, printing and binding costs,
telephone charges, postage, delivery service fees and other disbursements or
expenses of the types customarily incurred in connection with investigations,
judicial or administrative proceedings or appeals, but shall not include the
amount of judgments,
<PAGE>
fines or penalties against Indemnitee or amounts paid in settlement in
connection with such matters.

            (d) References to "other enterprise" shall include employee benefit
plans; references to "fines" shall include any excise tax assessed with respect
to any employee benefit plan; references to "serving at the request of the
Corporation" shall include any service as a director, officer, employee or agent
of the Corporation which imposes duties on, or involves services by, such
director, officer, employee, or agent with respect to an employee benefit plan,
its participants, or beneficiaries; and a person who acted in good faith and in
a manner such person reasonably believed to be in the interests of the
participants and beneficiaries of an employee benefit plan shall be deemed to
have acted in a manner "not opposed to the best interests of the Corporation" as
referred to in this Agreement.

      3. Indemnification in Third-Party Proceedings. The Corporation shall
indemnify the Indemnitee in accordance with the provisions of this Paragraph 3
if the Indemnitee was or is a party to or threatened to be made a party to or
otherwise involved in any Proceeding (other than a Proceeding by or in the right
of the Corporation to procure a judgment in its favor) by reason of the
Indemnitee's Corporate Status or by reason of any action alleged to have been
taken or omitted in connection therewith, against all Expenses, judgments,
fines, penalties and amounts paid in settlement actually and reasonably incurred
by or on behalf of the Indemnitee in connection with such Proceeding, if the
Indemnitee acted in good faith and in a manner which the Indemnitee reasonably
believed to be in, or not opposed to, the best interests of the Corporation and,
with respect to any criminal Proceeding, had no reasonable cause to believe that
his or her conduct was unlawful. The termination of any Proceeding by judgment,
order, settlement, conviction or upon a plea of nolo contendere or its
equivalent, shall not, of itself, create a presumption that the Indemnitee did
not act in good faith and in a manner which the Indemnitee reasonably believed
to be in, or not opposed to, the best interests of the Corporation, and, with
respect to any criminal Proceeding, had reasonable cause to believe that his or
her conduct was unlawful.

      4. Indemnification in Proceedings by or in the Right of the Corporation.
The Corporation shall indemnify the Indemnitee in accordance with the provisions
of this Paragraph 4 if the Indemnitee was or is a party to or threatened to be
made a party to or otherwise involved in any Proceeding by or in the right of
the Corporation to procure a judgment in its favor by reason of the Indemnitee's
Corporate Status or by reason of any action alleged to have been taken or
omitted in connection therewith, against all Expenses and, to the extent
permitted by law, amounts paid in settlement actually and reasonably incurred by
or on behalf of the Indemnitee in connection with such Proceeding, if the
Indemnitee acted in good faith and in a manner which the Indemnitee reasonably
believed to be in, or not opposed to, the best interests of the Corporation,
except that no indemnification shall be made under this Paragraph 4 in respect
of any claim, issue, or matter as to which the Indemnitee shall have been
adjudged to be liable to the Corporation, unless, and only to the extent, that
the Court of Chancery of Delaware or the court in which such action or suit was
brought shall determine upon application that, despite the adjudication of such
liability but in view of all the circumstances of the case, the Indemnitee is
fairly and reasonably entitled to indemnity for such Expenses as the Court of
Chancery or such other court shall deem proper.

                                      -2-
<PAGE>
      5. Exceptions to Right of Indemnification. Notwithstanding anything to the
contrary in this Agreement, except as set forth in Paragraph 10, the Corporation
shall not indemnify the Indemnitee in connection with a Proceeding (or part
thereof) initiated by the Indemnitee unless the initiation thereof was approved
by the Board of Directors of the Corporation. Notwithstanding anything to the
contrary in this Agreement, the Corporation shall not indemnify the Indemnitee
to the extent the Indemnitee is reimbursed from the proceeds of insurance, and
in the event the Corporation makes any indemnification payments to the
Indemnitee and the Indemnitee is subsequently reimbursed from the proceeds of
insurance, the Indemnitee shall promptly refund such indemnification payments to
the Corporation to the extent of such insurance reimbursement.

      6. Indemnification of Expenses of Successful Party. Notwithstanding any
other provision of this Agreement, to the extent that the Indemnitee has been
successful, on the merits or otherwise, in defense of any Proceeding or in
defense of any claim, issue or matter therein, the Indemnitee shall be
indemnified against all Expenses incurred by or on behalf of the Indemnitee in
connection therewith. Without limiting the foregoing, if any Proceeding or any
claim, issue or matter therein is disposed of, on the merits or otherwise
(including a disposition without prejudice), without (i) the disposition being
adverse to the Indemnitee, (ii) an adjudication that the Indemnitee was liable
to the Corporation, (iii) a plea of guilty or nolo contendere by the Indemnitee,
(iv) an adjudication that the Indemnitee did not act in good faith and in a
manner the Indemnitee reasonably believed to be in or not opposed to the best
interests of the Corporation, and (v) with respect to any criminal proceeding,
an adjudication that the Indemnitee had reasonable cause to believe his or her
conduct was unlawful, the Indemnitee shall be considered for the purposes hereof
to have been wholly successful with respect thereto.

      7. Notification and Defense of Claim. As a condition precedent to the
Indemnitee's right to be indemnified, the Indemnitee must notify the Corporation
in writing as soon as practicable of any Proceeding for which indemnity will or
could be sought. With respect to any Proceeding of which the Corporation is so
notified, the Corporation will be entitled to participate therein at its own
expense and/or, except as provided below, to assume the defense thereof at its
own expense, with legal counsel reasonably acceptable to the Indemnitee. After
notice from the Corporation to the Indemnitee of its election so to assume such
defense, the Corporation shall not be liable to the Indemnitee for any legal or
other expenses subsequently incurred by the Indemnitee in connection with such
Proceeding, other than as provided below in this Paragraph 7. The Indemnitee
shall have the right to employ his or her own counsel in connection with such
Proceeding, but the fees and expenses of such counsel incurred after notice from
the Corporation of its assumption of the defense thereof shall be at the expense
of the Indemnitee unless (i) the employment of counsel by the Indemnitee has
been authorized by the Corporation, (ii) counsel to the Indemnitee shall have
reasonably concluded that there may be a conflict of interest or position on any
significant issue between the Corporation and the Indemnitee in the conduct of
the defense of such Proceeding or (iii) the Corporation shall not in fact have
employed counsel to assume the defense of such Proceeding, in each of which
cases the fees and expenses of counsel for the Indemnitee shall be at the
expense of the Corporation, except as otherwise expressly provided by this
Agreement. The Corporation shall not be entitled, without the consent of the
Indemnitee, to assume the defense of any claim brought by or in the right of the
Corporation or as to which counsel for the Indemnitee shall have reasonably made
the conclusion provided for in clause (ii) above. The Corporation shall not be
required to indemnify

                                      -3-
<PAGE>
the Indemnitee under this Agreement for any amounts paid in settlement of any
Proceeding effected without its written consent. The Corporation shall not
settle any Proceeding in any manner which would impose any penalty or limitation
on the Indemnitee without the Indemnitee's written consent. Neither the
Corporation nor the Indemnitee will unreasonably withhold or delay their consent
to any proposed settlement.

      8. Advancement of Expenses. Subject to the provisions of Paragraph 9 of
this Agreement, in the event that the Corporation does not assume the defense
pursuant to Paragraph 7 of this Agreement of any Proceeding of which the
Corporation receives notice under this Agreement, any Expenses incurred by or on
behalf of the Indemnitee in defending such Proceeding shall be paid by the
Corporation in advance of the final disposition of such Proceeding; provided,
however, that the payment of such Expenses incurred by or on behalf of the
Indemnitee in advance of the final disposition of such Proceeding shall be made
only upon receipt of an undertaking by or on behalf of the Indemnitee to repay
all amounts so advanced in the event that it shall ultimately be determined that
the Indemnitee is not entitled to be indemnified by the Corporation as
authorized in this Agreement. Such undertaking shall be accepted without
reference to the financial ability of the Indemnitee to make repayment.

      9. Procedure for Indemnification. In order to obtain indemnification or
advancement of Expenses pursuant to Paragraphs 3, 4, 6 or 8 of this Agreement,
the Indemnitee shall submit to the Corporation a written request. Any such
indemnification or advancement of Expenses shall be made promptly, and in any
event within 30 days after receipt by the Corporation of the written request of
the Indemnitee, unless with respect to requests under Paragraphs 3, 4 or 8 the
Corporation determines within such 30-day period that the Indemnitee did not
meet the applicable standard of conduct set forth in Paragraph 3 or 4, as the
case may be. Such determination, and any determination that advanced Expenses
must be repaid to the Corporation, shall be made in each instance (a) by a
majority vote of the directors of the Corporation consisting of persons who are
not at that time parties to the Proceeding ("disinterested directors"), whether
or not a quorum, (b) by a committee of disinterested directors designated by a
majority vote of disinterested directors, whether or not a quorum, (c) if there
are no disinterested directors, or if the disinterested directors so direct, by
independent legal counsel (who may, to the extent permitted by applicable law,
be regular legal counsel to the Corporation) in a written opinion, or (d) by the
stockholders of the Corporation.

      10. Remedies. The right to indemnification or advancement of Expenses as
provided by this Agreement shall be enforceable by the Indemnitee in any court
of competent jurisdiction. Unless otherwise required by law, the burden of
proving that indemnification is not appropriate shall be on the Corporation.
Neither the failure of the Corporation to have made a determination prior to the
commencement of such action that indemnification is proper in the circumstances
because the Indemnitee has met the applicable standard of conduct, nor an actual
determination by the Corporation pursuant to Paragraph 9 that the Indemnitee has
not met such applicable standard of conduct, shall be a defense to the
Corporation's action or create a presumption that the Indemnitee has not met the
applicable standard of conduct. The Indemnitee's expenses (of the type described
in the definition of "Expenses" in Paragraph 2(c)) reasonably incurred in
connection with successfully establishing the Indemnitee's right to
indemnification, in whole or in part, in any such Proceeding shall also be
indemnified by the Corporation.

                                       -4-
<PAGE>
      11. Partial Indemnification. If the Indemnitee is entitled under any
provision of this Agreement to indemnification by the Corporation for some or a
portion of the Expenses, judgments, fines, penalties or amounts paid in
settlement actually and reasonably incurred by or on behalf of the Indemnitee in
connection with any Proceeding but not, however, for the total amount thereof,
the Corporation shall nevertheless indemnify the Indemnitee for the portion of
such Expenses, judgments, fines, penalties or amounts paid in settlement to
which the Indemnitee is entitled.

      12. Subrogation. In the event of any payment under this Agreement, the
Corporation shall be subrogated to the extent of such payment to all of the
rights of recovery of the Indemnitee, who shall execute all papers required and
take all action necessary to secure such rights, including execution of such
documents as are necessary to enable the Corporation to bring suit to enforce
such rights.

      13. Term of Agreement. This Agreement shall continue until and terminate
upon the later of (a) six years after the date that the Indemnitee shall have
ceased to serve as a director or officer of the Corporation or, at the request
of the Corporation, as a director, officer, partner, trustee, member, employee
or agent of another corporation, partnership, joint venture, trust, limited
liability company or other enterprise or (b) the final termination of all
Proceedings pending on the date set forth in clause (a) in respect of which the
Indemnitee is granted rights of indemnification or advancement of Expenses
hereunder and of any proceeding commenced by the Indemnitee pursuant to
Paragraph 10 of this Agreement relating thereto.

      14. Indemnification Hereunder Not Exclusive. The indemnification and
advancement of Expenses provided by this Agreement shall not be deemed exclusive
of any other rights to which the Indemnitee may be entitled under the
Certification of Incorporation, the By-Laws, any other agreement, any vote of
stockholders or disinterested directors, the General Corporation Law of
Delaware, any other law (common or statutory), or otherwise, both as to action
in the Indemnitee's official capacity and as to action in another capacity while
holding office for the Corporation. Nothing contained in this Agreement shall be
deemed to prohibit the Corporation from purchasing and maintaining insurance, at
its expense, to protect itself or the Indemnitee against any expense, liability
or loss incurred by it or the Indemnitee in any such capacity, or arising out of
the Indemnitee's status as such, whether or not the Indemnitee would be
indemnified against such expense, liability or loss under this Agreement;
provided that the Corporation shall not be liable under this Agreement to make
any payment of amounts otherwise indemnifiable hereunder if and to the extent
that the Indemnitee has otherwise actually received such payment under any
insurance policy, contract, agreement or otherwise.

      15. No Special Rights. Nothing herein shall confer upon the Indemnitee any
right to continue to serve as an officer or director of the Corporation for any
period of time or at any particular rate of compensation.

      16. Savings Clause. If this Agreement or any portion thereof shall be
invalidated on any ground by any court of competent jurisdiction, then the
Corporation shall nevertheless indemnify the Indemnitee as to Expenses,
judgments, fines, penalties and amounts paid in settlement with respect to any
Proceeding to the full extent permitted by any applicable portion

                                      -5-
<PAGE>
of this Agreement that shall not have been invalidated and to the fullest extent
permitted by applicable law.

      17. Counterparts. This Agreement may be executed in any number of
counterparts, each of which shall constitute the original.

      18. Successors and Assigns. This Agreement shall be binding upon the
Corporation and its successors and assigns and shall inure to the benefit of the
estate, heirs, executors, administrators and personal representatives of the
Indemnitee.

      19. Headings. The headings of the paragraphs of this Agreement are
inserted for convenience only and shall not be deemed to constitute part of this
Agreement or to affect the construction thereof.

      20. Modification and Waiver. This Agreement may be amended from time to
time to reflect changes in Delaware law or for other reasons. No supplement,
modification or amendment of this Agreement shall be binding unless executed in
writing by both of the parties hereto. No waiver of any of the provisions of
this Agreement shall be deemed or shall constitute a waiver of any other
provision hereof nor shall any such waiver constitute a continuing waiver.

      21. Notices. All notices, requests, demands and other communications
hereunder shall be in writing and shall be deemed to have been given (i) when
delivered by hand or (ii) if mailed by certified or registered mail with postage
prepaid, on the third day after the date on which it is so mailed:

            (a) if to the Indemnitee, to:

            (b) if to the Corporation, to:         NxStage Medical, Inc.
                                                   439 South Union Street
                                                   Lawrence, MA 01843
                                                   Attention:  Chief Financial
                                                               Officer

                with a copy to:                    NxStage Medical, Inc.
                                                   439 South Union Street
                                                   Lawrence, MA 01843
                                                   Attention:  General Counsel

or to such other address as may have been furnished to the Indemnitee by the
Corporation or to the Corporation by the Indemnitee, as the case may be.

      22. Applicable Law. This Agreement shall be governed by, and construed and
enforced in accordance with, the laws of the State of Delaware. The Indemnitee
may elect to have the right to indemnification or reimbursement or advancement
of Expenses interpreted on the basis of the applicable law in effect at the time
of the occurrence of the event or events giving rise to the applicable
Proceeding, to the extent permitted by law, or on the basis of the applicable
law in effect at the time such indemnification or reimbursement or advancement
of Expenses is

                                      -6-
<PAGE>
sought. Such election shall be made, by a notice in writing to the Corporation,
at the time indemnification or reimbursement or advancement of Expenses is
sought; provided, however, that if no such notice is given, and if the General
Corporation Law of Delaware is amended, or other Delaware law is enacted, to
permit further indemnification of the directors and officers, then the
Indemnitee shall be indemnified to the fullest extent permitted under the
General Corporation Law, as so amended, or by such other Delaware law, as so
enacted.

      23. Enforcement. The Corporation expressly confirms and agrees that it has
entered into this Agreement in order to induce the Indemnitee to continue to
serve as an officer or director of the Corporation, and acknowledges that the
Indemnitee is relying upon this Agreement in continuing in such capacity.

      24. Entire Agreement. This Agreement sets forth the entire agreement of
the parties hereto in respect of the subject matter contained herein and
supercedes all prior agreements, whether oral or written, by any officer,
employee or representative of any party hereto in respect of the subject matter
contained herein; and any prior agreement of the parties hereto in respect of
the subject matter contained herein is hereby terminated and cancelled. For
avoidance of doubt, the parties confirm that the foregoing does not apply to or
limit the Indemnitee's rights under Delaware law or the Corporation's
Certificate of Incorporation or By-Laws.

      25. Consent to Suit. In the case of any dispute under or in connection
with this Agreement, the Indemnitee may only bring suit against the Corporation
in the Court of Chancery of the State of Delaware. The Indemnitee hereby
consents to the exclusive jurisdiction and venue of the courts of the State of
Delaware, and the Indemnitee hereby waives any claim the Indemnitee may have at
any time as to forum non conveniens with respect to such venue. The Corporation
shall have the right to institute any legal action arising out of or relating to
this Agreement in any court of competent jurisdiction. Any judgment entered
against either of the parties in any proceeding hereunder may be entered and
enforced by any court of competent jurisdiction.

IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be duly
executed as of the day and year first above written.

                                              NXSTAGE MEDICAL, INC.

Attest:                                       By:    ________________________
By:      _______________________              Name:  ________________________
Name: __________________________              Title: ________________________

                                              INDEMNITEE:

                                              ------------------------------
                                              Name:

                                      -7-

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