Document:

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                                                                    Exhibit 10.7

THIS WARRANT HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS
AMENDED (THE "1933 ACT"), OR ANY APPLICABLE STATE SECURITIES LAWS, AND MAY NOT
BE SOLD OR TRANSFERRED UNLESS SUCH SALE OR TRANSFER IS IN ACCORDANCE WITH THE
REGISTRATION REQUIREMENTS OF SUCH ACT AND APPLICABLE LAWS OR SOME OTHER
EXEMPTION FROM THE REGISTRATION REQUIREMENTS OF SUCH ACT AND APPLICABLE LAWS IS
AVAILABLE WITH RESPECT THERETO.

                        PREFERRED STOCK PURCHASE WARRANT

Warrant No._________       Number of Shares:  To be determined pursuant to
                             SECTION 2 below of Series F Convertible Preferred
                             Stock, $0.001 par value per share

                              NXSTAGE MEDICAL, INC.

                        Effective as of December 23, 2004

                          Void after December 23, 2011

      1. ISSUANCE. This Preferred Stock Purchase Warrant (the "Warrant") is
issued to LIGHTHOUSE CAPITAL PARTNERS V, L.P., or its registered assigns (the
"Holder"), by NXSTAGE MEDICAL, INC., a Delaware corporation (hereinafter with
its successors called the "Company").

      2. PURCHASE PRICE; NUMBER OF SHARES. The Holder commencing on the date
hereof is entitled upon surrender of this Warrant with the subscription form
annexed hereto duly executed, at the principal office of the Company, to
purchase from the Company that number of fully paid and nonassessable shares
(the "Shares") of Series F Preferred Stock, $.0.001 par value, of the Company
(the "Preferred Stock") equal to $300,000 divided by the Purchase Price (as
defined below). As used in this Warrant, the "Purchase Price" shall equal the
lesser of (i) $7.28 (the per share price of the Preferred Stock), or (ii) the
price per share of securities issued in an Equity Infusion, if such Equity
Infusion occurs prior to the exercise of this Warrant, provided that:

            (a) if the Equity Infusion is an initial Public Offering (as defined
below), the price per share of securities is the price to the public of the
shares of the Company's Common Stock, $0.01 par value per share (the "Common
Stock"), issued and sold in the offering, and

            (b) if the Equity Infusion is a Next Round Financing (as defined
below), the price per share of equity securities is the price at which shares of
the Company's preferred stock are issued in such Next Round Financing,

            (c) in the case of an Equity Infusion other than an initial Public
Offering or a Next Round Financing, the price per share of securities is the
fair market value (as determined by the Board of Directors of the Company) of
the consideration per share of Preferred Stock issued in the Equity Infusion.

Notwithstanding the foregoing, the Purchase Price shall in no event be less than
$5.97 per share.

In addition to other terms which may be defined herein, the following terms, as
used in this Warrant, shall have the following meanings:

                  (i) "Equity Infusion" shall mean the first to occur of the
Company's (a) Next Round Financing, (b) initial Public Offering, or (c)
acquisition by or merger into another company, or the sale of all or
substantially all assets to another company (other than a merger or
consolidation in which all or substantially all of the individuals and entities
who were beneficial owners of the capital stock immediately prior to such
transaction beneficially own, directly or indirectly, more than 50% of the
outstanding securities entitled to vote generally in the election of directors
of the resulting, surviving or acquiring corporation in such transaction).

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                  (ii) "Next Round Financing" means the Company's next bona fide
round of preferred stock equity financing for the purpose of raising working
capital for the Company with gross aggregate proceeds to the Company of at least
$3,000,000.

Until such time as this Warrant is exercised in full or expires, the Purchase
Price and the securities issuable upon exercise of this Warrant are subject to
adjustment as hereinafter provided.

      3. PAYMENT OF PURCHASE PRICE; EXERCISE DATE.

            (a) Subject to SECTION 4 below, the Purchase Price may be paid (i)
in cash or by check, (ii) by the surrender by the Holder to the Company of any
promissory notes or other obligations issued by the Company, with all such notes
and obligations so surrendered being credited against the Purchase Price in an
amount equal to the principal amount thereof plus accrued interest to the date
of surrender, or (iii) by any combination of the foregoing.

            (b) Each exercise of this Warrant shall be deemed to have been
effected immediately prior to the close of business on the day on which this
Warrant shall have been surrendered to the Company as provided in subsection
3(a) above or SECTION 4 below the ("Exercise Date"). At such time, the person or
persons in whose name or names any certificates for Shares shall be issuable
upon such exercise shall be deemed to have become the Holder or Holders of
record of the Shares represented by such certificates.

      4. NET ISSUE ELECTION. The Holder may elect to receive, without the
payment by the Holder of any additional consideration, shares of Preferred Stock
equal to the value of this Warrant or any portion hereof by the surrender of
this Warrant or such portion to the Company, with the net issue election notice
annexed hereto duly executed, at the principal office of the Company. Thereupon,
the Company shall issue to the Holder such number of fully paid and
nonassessable shares of Preferred Stock as is computed using the following
formula:

                                       Y(A-B)
                                  X = --------
                                         A

      Where: X =  the number of shares of Preferred Stock to be issued to the
                  Holder pursuant to this SECTION 4.

             Y =  the number of shares of Preferred Stock covered by this
                  Warrant in respect of which the net issue election is made
                  pursuant to this SECTION 4.

             A =  the Fair Market Value (defined below) of one share of
                  Preferred Stock, as determined at the time the net issue
                  election is made pursuant to this SECTION 4.

             B =  the Purchase Price in effect under this Warrant at the time
                  the net issue election is made pursuant to this SECTION 4.

            "Fair Market Value" of a share of Preferred Stock (or fully paid and
nonassessable shares of the Company's Common Stock if the Preferred Stock has
been automatically converted into Common Stock) as of the date that the net
issue election is made (the "Determination Date") shall mean:

                  (i) If the net issue election is made in connection with and
contingent upon the closing of the initial sale of the Company's Common Stock to
the public in a public offering pursuant to a Registration Statement under the
1933 Act (a "Public Offering"), and if the Company's Registration Statement
relating to such Public Offering ("Registration Statement") has been declared
effective by the Securities and Exchange Commission, then the initial "Price to
Public" specified in the final prospectus with respect to such offering
multiplied by the number of shares of Common Stock into which each share of
Preferred Stock is then convertible.

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                  (ii) If the net issue election is not made in connection with
and contingent upon a Public Offering, then as follows:

                        (a) If traded on a securities exchange, the NASDAQ
National Market or another nationally recognized trading system as of the
Exercise Date, the fair market value of the Common Stock shall be deemed to be
the average of the closing or last reported sale prices of the Common Stock on
such exchange or market over the five day period ending five trading days prior
to the Determination Date, and the fair market value of the Preferred Stock
shall be deemed to be such fair market value of the Common Stock multiplied by
the number of shares of Common Stock into which each share of Preferred Stock is
then convertible;

                        (b) If otherwise traded in an over-the-counter market,
the fair market value of the Common Stock shall be deemed to be the average of
the closing ask prices of the Common Stock over the five day period ending five
trading days prior to the Determination Date, and the fair market value of the
Preferred Stock shall be deemed to be such fair market value of the Common Stock
multiplied by the number of shares of Common Stock into which each share of
Preferred Stock is then convertible; and

                        (c) If there is no public market for the Common Stock,
then fair market value shall be determined in good faith by the Company's Board
of Directors.

      5. PARTIAL EXERCISE. This Warrant may be exercised in part, and the Holder
shall be entitled to receive a new warrant, which shall be dated as of the date
of this Warrant, covering the number of shares in respect of which this Warrant
shall not have been exercised.

      6. FRACTIONAL SHARES. In no event shall any fractional share of Preferred
Stock be issued upon any exercise of this Warrant. If, upon exercise of this
Warrant in its entirety, the Holder would, except as provided in this SECTION 6,
be entitled to receive a fractional share of Preferred Stock, then the Company
shall pay to the Holder the value thereof in cash on the basis of Fair Market
Value as determined in SECTION 4 above.

      7. EXPIRATION DATE; AUTOMATIC EXERCISE. This Warrant shall expire at the
close of business on December 23, 2011 (the "Expiration Date") and shall be void
thereafter.

Notwithstanding the foregoing, this Warrant shall automatically be deemed to be
exercised in full pursuant to the provisions of SECTION 4 hereof, without any
further action on behalf of the Holder, immediately prior to the time this
Warrant would otherwise expire pursuant to this SECTION 7.

      8. RESERVED SHARES; VALID ISSUANCE. The Company covenants that it will at
all times from and after the date hereof reserve and keep available such number
of its authorized shares of Preferred Stock and Common Stock free from all
preemptive or similar rights therein, as will be sufficient to permit,
respectively, the exercise of this Warrant in full and the conversion into
shares of Common Stock of all shares of Preferred Stock receivable upon such
exercise. The Company further covenants that such shares as may be issued
pursuant to such exercise and/or conversion will, upon issuance, be duly and
validly issued, fully paid and nonassessable and free from all taxes, liens and
charges with respect to the issuance thereof.

      9. STOCK SPLITS AND DIVIDENDS. If after the date hereof the Company shall
subdivide the Preferred Stock, by split-up or otherwise, or combine the
Preferred Stock, or issue additional shares of Preferred Stock in payment of a
stock dividend on the Preferred Stock, the number of shares of Preferred Stock
issuable on the exercise of this Warrant shall forthwith be proportionately
increased in the case of a subdivision or stock dividend, or proportionately
decreased in the case of a combination, and the Purchase Price shall forthwith
be proportionately decreased in the case of a subdivision or stock dividend, or
proportionately increased in the case of a combination. Any adjustment under
this SECTION 9 shall become effective at the close of business on the date the
dividend, subdivision or combination becomes effective.

      10. ADJUSTMENTS FOR DILUTING ISSUANCES. The other antidilution rights
applicable to the Preferred Stock and the Common Stock of the Company, if any,
are set forth in the Amended and Restated Certificate of

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Incorporation, as amended from time to time (the "Certificate"), a true and
complete copy in its current form which is attached hereto as EXHIBIT A. The
Company shall promptly provide the Holder hereof with any restatement, amendment
or modification to the Certificate promptly after the same has been made.

      11. MERGERS AND RECLASSIFICATIONS. If after the date hereof the Company
shall enter into any Reorganization (as hereinafter defined), then, following
such Reorganization, the Holder shall receive upon exercise hereof the kind and
amount of securities, cash or other property which the Holder would have been
entitled to receive pursuant to such Reorganization if such exercise had taken
place immediately prior to such Reorganization. In addition, as a condition of
such Reorganization, lawful provisions shall be made, and duly executed
documents evidencing the same from the Company or its successor shall be
delivered to the Holder, with respect to the rights and interest of the Holder
to the end that the provisions hereof (including without limitation, provisions
for the adjustment of the Purchase Price and the number of shares issuable
hereunder and the provisions relating to the net issue election) shall
thereafter be applicable in relation to any shares of stock or other securities
and property thereafter deliverable upon exercise hereof. For the purposes of
this SECTION 11, the term "Reorganization" shall include without limitation any
reclassification, capital reorganization or change of the Preferred Stock (other
than as a result of a subdivision, combination or stock dividend provided for in
Section 9 hereof), or any consolidation of the Company with, or merger of the
Company into, another corporation or other business organization (other than a
merger in which the Company is the surviving corporation and which does not
result in any reclassification or change of the outstanding Preferred Stock), or
any sale or conveyance to another corporation or other business organization of
all or substantially all of the assets of the Company in which the Preferred
Stock is converted or exchanged for securities, cash or other property.

      12. CERTIFICATE OF ADJUSTMENT. Whenever the Purchase Price is adjusted, as
herein provided, the Company shall promptly deliver to the Holder a certificate
of the Company's chief financial officer setting forth the Purchase Price after
such adjustment and setting forth a brief statement of the facts requiring such
adjustment.

      13. NOTICES OF RECORD DATE, ETC. In the event of:

            (a) any taking by the Company of a record of the holders of any
class of securities for the purpose of determining the holders thereof who are
entitled to receive any dividend or other distribution, or any right to
subscribe for, purchase, sell or otherwise acquire or dispose of any shares of
stock of any class or any other securities or property, or to receive any other
right;

            (b) any reclassification of the capital stock of the Company,
capital reorganization of the Company, consolidation or merger involving the
Company, or sale or conveyance of all or substantially all of its assets (other
than a consolidation or merger in which the Company is the surviving company and
its capital stock is not converted or exchanged for any other securities or
property); or

            (c) any voluntary or involuntary dissolution, liquidation or
winding-up of the Company;

then in each such event the Company will provide or cause to be provided to the
Holder a written notice thereof. Such notice shall be provided at least 10 days
prior to the date specified in such notice on which any such action is to be
taken.

      14. REPRESENTATIONS, WARRANTIES AND COVENANTS. This Warrant is issued and
delivered by the Company and accepted by each Holder on the basis of the
following representations, warranties and covenants made by the Company:

            (a) The Company has all necessary authority to issue, execute and
deliver this Warrant and to perform its obligations hereunder. This Warrant has
been duly authorized issued, executed and delivered by the Company and is the
valid and binding obligation of the Company, enforceable in accordance with its
terms.

            (b) The shares of Preferred Stock issuable upon the exercise of this
Warrant as of the date of issuance of this Warrant have been duly authorized and
reserved for issuance by the Company and, when issued in

                                      -4-
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accordance with the terms hereof, will be validly issued, fully paid and
nonassessable. If, as a result of an adjustment in the Purchase Price or other
adjustment herein, the number of shares of Preferred Stock issuable upon
exercise of this Warrant increases, then the Company will undertake to duly
authorize and reserve sufficient shares for issuance by the Company, and such
shares, when issued in accordance with the terms hereof, will be validly issued,
fully paid and nonassessable.

            (c) The issuance, execution and delivery of this Warrant do not, and
the issuance of the shares of Preferred Stock upon the exercise of this Warrant
in accordance with the terms hereof will not, (i) violate or contravene the
Company's Certificate or by-laws, or any law, statute, regulation, rule,
judgment or order applicable to the Company, (ii) violate, contravene or result
in a breach or default under any contract, agreement or instrument to which the
Company is a party or by which the Company or any of its assets are bound or
(iii) require the consent or approval of or the filing of any notice or
registration with any person or entity.

            (d) So long as this Warrant has not terminated, Holder shall be
entitled to receive such financial and other information as the Holder would be
entitled to receive under the Stock Purchase Agreement applicable to the
Preferred Stock if Holder were a holder of that number of shares issuable upon
full exercise of this Warrant. In addition, upon exercise of this Warrant,
Holder shall be entitled to receive such financial and other information as the
Holder would be entitled to receive under the Stock Purchase Agreement
applicable to the Preferred Stock if such Holder purchased such number of shares
of Preferred Stock under such Stock Purchase Agreement.

            (e) As of the date hereof, the authorized capital stock of the
Company consists of (i) 20,000,000 shares of Common Stock, of which 3,510,283
shares are issued and outstanding and 82,418 shares are reserved for issuance
upon the exercise of this Warrant with respect to Common Stock and the
conversion of the Preferred Stock into Common Stock if this Warrant is exercised
with respect to Preferred Stock, (ii) 1,875,000 shares of Series B Preferred
Stock, of which 1,875,000 are issued and outstanding shares, (iii) 1,155,169
shares of Series C Preferred Stock, of which 1,151,632 are issued and
outstanding shares, (iv) 5,011,173 shares of Series D Preferred Stock, of which
4,857,622 are issued and outstanding shares, (v) 2,690,846 shares of Series E
Preferred Stock, of which 2,690,846 are issued and outstanding shares, and (vi)
2,829,671 shares of Series F Preferred Stock, of which 2,747,253 are issued and
outstanding shares. Attached hereto as EXHIBIT B is a capitalization table
summarizing the capitalization of the Company. Once per calendar quarter, the
Company will provide Holder with a current capitalization table indicating
changes, if any, to the number of outstanding shares of common stock and
preferred stock.

      15. REGISTRATION RIGHTS. The Company grants to the Holder all the rights
of a "Holder" and an "Investor" under the Company's Investors' Rights Agreement,
dated as of August 18, 2004 (the "Rights Agreement"), including, without
limitation, the registration rights contained therein, and agrees to amend the
Rights Agreement in such form as is approved by the Company and Lighthouse
Capital Partners V, L.P. so that (i) the shares of Common Stock issuable upon
conversion of the shares of Preferred Stock issuable upon exercise of this
Warrant shall be "Registrable Securities," as amended, and (ii) the Holder shall
be a "Holder" and an "Investor" for all purposes of such Rights Agreement.

      16. AMENDMENT. The terms of this Warrant may be amended, modified or
waived only with the written consent of the Holder and the Company.

      17. REPRESENTATIONS AND COVENANTS OF THE HOLDER. This Warrant has been
entered into by the Company in reliance upon the following representations and
covenants of the Holder, which by its execution hereof the Holder hereby
confirms:

            (a) INVESTMENT PURPOSE. The Holder is acquiring the Warrant and (if
and when it exercises this Warrant), it will acquire Preferred Stock for
investment and not with a view to the sale or distribution of any part thereof,
and the Holder has no present intention of selling or engaging in any public
distribution of the same except pursuant to a registration or exemption, and the
Holder has no present or contemplated agreement, undertaking, arrangement,
obligation, indebtedness or commitment providing for the distribution thereof.

                                      -5-
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            (b) ACCREDITED INVESTOR. Holder is an "accredited investor" within
the meaning of the Securities and Exchange Rule 501 of Regulation D under the
1933 Act, as presently in effect.

            (c) PRIVATE ISSUE. The Holder understands (i) that the Preferred
Stock issuable upon exercise of the Holder's rights contained herein is not
registered under the 1933 Act or qualified under applicable state securities
laws on the ground that the issuance contemplated by this Warrant will be exempt
from the registration and qualifications requirements thereof, and (ii) that the
Company's reliance on such exemption is predicated on the representations set
forth in this SECTION 17.

            (d) FINANCIAL RISK. The Holder has such knowledge and experience in
financial and business matters as to be capable of evaluating the merits and
risks of its investment and has the ability to bear the economic risks of its
investment. The Holder has made such inquiry concerning the Company and its
business and personnel as it has deemed appropriate.

      18. NOTICES, TRANSFERS, ETC.

            (a) Any notice or written communication required or permitted to be
given to the Holder may be given by certified or registered mail, postage
prepaid, or sent via a reputable nationwide overnight courier service
guaranteeing next business day delivery, or delivered by hand to the Holder at
the address most recently provided by the Holder to the Company. All notices and
other communications from the Holder to the Company in connection herewith shall
be mailed by certified or registered mail, postage prepaid, or sent via a
reputable nationwide overnight courier service guaranteeing next business day
delivery, to the Company at its principal office set forth below. All such
notices and communications shall be deemed delivered (i) two business days after
being sent by certified or registered mail, return receipt requested, postage
prepaid, or (ii) one business day after being sent via a reputable nationwide
overnight courier service guaranteeing next business day delivery.

            (b) Subject to compliance with applicable federal and state
securities laws, and to the extent requested by the Company, receipt by the
Company of an opinion of legal counsel, reasonably satisfactory to the Company,
to the effect that such transfer is exempt from the registration requirements of
the 1933 Act, this Warrant may be transferred by the Holder with respect to any
or all of the shares purchasable hereunder. Subject to the first sentence of
this SECTION 18, upon surrender of this Warrant to the principal offices of the
Company (or such other office or agency designated by the Company), together
with the assignment notice annexed hereto duly executed, for transfer of this
Warrant as an entirety by the Holder and upon payment by the Holder of any
applicable transfer taxes, the Company shall issue a new warrant of the same
denomination to the assignee. Subject to the first sentence of this SECTION 18,
upon surrender of this Warrant to the principal offices of the Company (or such
other office or agency designated by the Company), together with the assignment
hereof properly endorsed, by the Holder for transfer with respect to a portion
of the shares of Preferred Stock purchasable hereunder, and upon payment by the
Holder of any applicable transfer taxes, the Company shall issue a new warrant
to the assignee, in such denomination as shall be requested by the Holder
hereof, and shall issue to such Holder a new warrant covering the number of
shares in respect of which this Warrant shall not have been transferred.

            (c) In case this Warrant shall be mutilated, lost, stolen or
destroyed, the Company shall issue a new warrant of like tenor and denomination
and deliver the same (i) in exchange and substitution for and upon surrender and
cancellation of any mutilated Warrant, or (ii) in lieu of any Warrant lost,
stolen or destroyed, upon receipt of an affidavit of the Holder or other
evidence reasonably satisfactory to the Company of the loss, theft or
destruction of such Warrant.

            (d) Each certificate representing Shares shall bear a legend
substantially in the following form:

                  "The securities represented by this certificate have not been
                  registered under the Securities Act of 1933, as amended, and
                  may not be offered, sold or otherwise transferred, pledged or
                  hypothecated unless and until such securities are registered
                  under

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                  such Act or an opinion of counsel satisfactory to the Company
                  is obtained to the effect that such registration is not
                  required."

      The foregoing legend shall be removed from the certificates representing
any Shares, at the request of the holder thereof, at such time as they become
eligible for resale pursuant to Rule 144(k) under the Securities Act of 1933.

            (e) The Company will maintain a register containing the name and
address of the Holder of this Warrant. The Holder may change its address as
shown on the warrant register by written notice to the Company requesting such
change.

      19. NO IMPAIRMENT. The Company will not, by amendment of its Certificate
or through any reclassification, capital reorganization, consolidation, merger,
sale or conveyance of assets, dissolution, liquidation, issue or sale of
securities or any other voluntary action, avoid or seek to avoid the observance
of performance of any of the terms of this Warrant, but will at all times in
good faith assist in the carrying out of all such terms and in the taking of all
such action as may be necessary or appropriate in order to protect the rights of
the Holder.

      20. GOVERNING LAW. The provisions and terms of this Warrant shall be
governed by and construed in accordance with the internal laws of the State of
Delaware without giving effect to its principles regarding conflicts of laws.

      21. SUCCESSORS AND ASSIGNS. This Warrant shall be binding upon the
Company's successors and assigns and shall inure to the benefit of the Holder's
successors, legal representatives and permitted assigns.

      22. BUSINESS DAYS. If the last or appointed day for the taking of any
action required or the expiration of any rights granted herein shall be a
Saturday or Sunday or a legal holiday in California, then such action may be
taken or right may be exercised on the next succeeding day which is not a
Saturday or Sunday or such a legal holiday.

      23. ADJUSTMENT FOR CONVERSION OF PREFERRED STOCK. If all of the
outstanding shares of Preferred Stock are converted into Common Stock of the
Company in accordance with the terms of the Certificate then, effective upon
such conversion, (i) this Warrant shall be exercisable for such number of shares
of Common Stock as is equal to the number of shares of Common Stock that each
share of Preferred Stock was converted into, multiplied by the number of shares
of Preferred Stock subject to this Warrant immediately prior to such conversion,
(ii) the Purchase Price shall be the Purchase Price in effect immediately prior
to such conversion divided by the number of shares of Common Stock into which
each share of Preferred Stock was converted, and (iii) all references in this
Warrant to "Preferred Stock" shall thereafter be deemed to refer to "Common
Stock."

      24. VALUE. The Company and the Holder agree that the value of this Warrant
on the date of grant is $50.

                                             NXSTAGE MEDICAL, INC.

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

                                             Name:  Jeffrey H. Burbank

                                             Title:  President and CEO

                                      -7-<PAGE>

                                                                    Exhibit 10.8

THIS WARRANT AND THE SHARES ISSUABLE HEREUNDER HAVE NOT BEEN REGISTERED UNDER
THE SECURITIES ACT OF 1933, AS AMENDED, AND APPLICABLE STATE SECURITIES AND
LAWS, AND MAY NOT BE SOLD, PLEDGED OR OTHERWISE TRANSFERRED WITHOUT AN EFFECTIVE
REGISTRATION THEREOF UNDER SUCH ACT AND LAWS OR PURSUANT TO RULE 144 OR AN
OPINION OF COUNSEL REASONABLY SATISFACTORY TO THE CORPORATION AND ITS COUNSEL
THAT SUCH REGISTRATION IS NOT REQUIRED.

                            WARRANT TO PURCHASE STOCK

<TABLE>
<S>                      <C>
Corporation:             NxStage Medical, Inc., a Delaware corporation
Number of Shares:        20,938
Class of Stock:          SERIES E PREFERRED (Subject to Section 1.7)
Initial Exercise Price:  $5.97 per share
Issue Date:              September 26, 2002
Expiration Date:         September 26, 2009 (subject to Section 4.1)
</TABLE>

      THIS WARRANT CERTIFIES THAT, for good and valuable consideration, the
receipt of which is hereby acknowledged, COMERICA BANK - CALIFORNIA or its
assignee ("Holder") is entitled to purchase the number of fully paid and
nonassessable shares of the class of securities (the "Shares") of the
corporation (the "Company") at the initial exercise price per Share (the
"Warrant Price") all as set forth above and as adjusted pursuant to Article 2 of
this warrant, subject to the provisions and upon the terms and conditions set
forth in this warrant.

ARTICLE 1. EXERCISE.

      1.1 Method of Exercise. Holder may exercise this warrant by delivering
this warrant and a duly executed Notice of Exercise in substantially the form
attached as Appendix I to the principal office of the Company. Unless Holder is
exercising the conversion right set forth in Section 1.2, Holder shall also
deliver to the Company a check for the aggregate Warrant Price for the Shares
being purchased.

      1.2 Conversion Right. In lieu of exercising this warrant as specified in
Section 1.1, Holder may from time to time convert this warrant, in whole or in
part, into a number of Shares determined by dividing (a) the aggregate fair
market value of the Shares or other securities otherwise issuable upon exercise
of this warrant minus the aggregate Warrant Price of such Shares by (b) the fair
market value of one Share. The fair market value of the Shares shall be
determined pursuant to Section 1.3.

      1.3 Fair Market Value. If the Shares are traded regularly in a public
market, the fair market value of the Shares shall be the closing price of the
Shares (or the closing price of the Company's stock into which the Shares are
convertible) reported for the business day immediately before Holder delivers
its Notice of Exercise to the Company. If the Shares are not regularly traded in
a public market; the Board of Directors of the Company shall determine fair
market value in its reasonable good faith judgment.

<PAGE>

      1.4 Delivery of Certificate and New Warrant. Promptly after Holder
exercises or converts this warrant, the Company shall deliver to Holder
certificates for the Shares acquired and, if this warrant has not been fully
exercised or converted and has not expired, a new warrant representing the
Shares not so acquired.

      1.5 Replacement of Warrants. On receipt of evidence reasonably
satisfactory to the Company of the loss, theft, destruction or mutilation of
this warrant and, in the case of loss, theft or destruction, on delivery of an
indemnity agreement reasonably satisfactory in form and amount to the Company
or, in the case of mutilation, on surrender and cancellation of this warrant,
the Company at its expense shall execute and deliver, in lieu of this warrant, a
new warrant of like tenor.

      1.6 Repurchase on Sale, Merger, or Consolidation of the Company.

            1.6.1 "Acquisition." For the purpose of this warrant, "Acquisition"
means any sale, license, or other disposition of all or substantially all of the
assets (including intellectual property) of the Company, or any reorganization,
consolidation, or merger of the Company where the holders of the Company's
securities before the transaction beneficially own less than 50% of the
outstanding voting securities of the surviving entity after the transaction.

            1.6.2 Assumption of Warrant. If upon the closing of any Acquisition
the successor entity assumes the obligations of this warrant, then this warrant
shall be exercisable for the same securities, cash, and property as would be
payable for the Shares issuable upon exercise of the unexercised portion of this
warrant as if such Shares were outstanding on the record date for the
Acquisition and subsequent closing. The Warrant Price shall be adjusted
accordingly. The Company shall use reasonable efforts to cause the surviving
corporation to assume the obligations of this warrant.

            1.6.3 Nonassumption. If upon the closing of any Acquisition the
successor entity does not assume the obligations of this warrant and Holder has
not otherwise exercised this warrant in full, then Holder shall have the option
either to (a) deem this warrant to have been automatically converted pursuant to
Section 1.2 and thereafter Holder shall participate in the Acquisition on the
same terms as other holders of the same class of securities of the Company; or
(b) require the Company to purchase this warrant for cash upon the closing of
the Acquisition for an amount per Share equal to two (2) times the Warrant
Price.

      1.7 Change in Series of Preferred Stock. In the event that the Company (i)
does not close and receive proceeds from the issuance of its Series E Preferred
stock equity round on or before May 31, 2003 or (ii) enters into an Acquisition
(as defined in Section 1.6.1 of this warrant) or a public issuance of its equity
securities prior to that date, the Company hereby agrees that (A) it shall
increase the number of shares of Series D Preferred Stock to allow for the
conversion of this warrant into Series D Preferred shares, contingent upon
receipt of all necessary stockholder approvals, and (B) change the class of
securities into which this Warrant may be converted from Series E Preferred
Stock to Series D Preferred Stock. The Company agrees to use its best efforts to
obtain all needed stockholder approvals to increase the number of Series D
Preferred Stock available for issuance of this Warrant.

                                       2
<PAGE>

ARTICLE 2. ADJUSTMENTS TO THE SHARES.

      2.1 Stock Dividends, Splits, Etc. If the Company declares or pays a
dividend on its common stock payable in common stock, or other securities, or
subdivides the outstanding common stock into a greater amount of common stock,
then upon exercise of this warrant, for each Share acquired, Holder shall
receive, without cost to Holder, the total number and kind of securities to
which Holder would have been entitled had Holder owned the Shares of record as
of the date the dividend or subdivision occurred.

      2.2 Reclassification, Exchange or Substitution. Upon any reclassification,
exchange, substitution, or other event that results in a change of the number
and/or class of the securities issuable upon exercise or conversion of this
warrant, Holder shall be entitled to receive, upon exercise or conversion of
this warrant, the number and kind of securities and property that Holder would
have received for the Shares if this warrant had been exercised immediately
before such reclassification, exchange, substitution, or other event. Such an
event shall include any automatic conversion of the outstanding or issuable
securities of the Company of the same class or series as the Shares to common
stock pursuant to the terms of the Company's Certificate of Incorporation upon
the closing of a registered public offering of the Company's common stock. The
Company or its successor shall promptly issue to Holder a new warrant for such
new securities or other property. The new warrant shall provide for adjustments
which shall be as nearly equivalent as may be practicable to the adjustments
provided for in this Article 2 including, without limitation, adjustments to the
Warrant Price and to the number of securities or property issuable upon exercise
of the new warrant. The provisions of this Section 2.2 shall similarly apply to
successive reclassifications, exchanges, substitutions, or other events.

      2.3 Adjustments for Combinations, Etc. If the outstanding Shares are
combined or consolidated, by reclassification or otherwise, into a lesser number
of shares, the Warrant Price shall be proportionately increased. If the
outstanding Shares are combined or consolidated, by reclassification or
otherwise, into a greater number of shares, the Warrant Price shall be
proportionately decreased.

      2.4 Adjustments for Diluting Issuances. The Warrant Price and the number
of Shares issuable upon exercise of this warrant shall be subject to adjustment,
from time to time, in the manner set forth on Exhibit A in the event of Diluting
Issuances (as defined on Exhibit A).

      2.5 No Impairment. The Company shall not, by amendment of its Certificate
of Incorporation or through a reorganization, transfer of assets, consolidation,
merger, dissolution, issue, or sale of securities or any other voluntary action,
avoid or seek to avoid the observance or performance of any of the terms to be
observed or performed under this warrant by the Company, but shall at all times
in good faith assist in carrying out all the provisions of this Article 2 and in
taking all such action as may be necessary or appropriate to protect Holder's
rights under this Article against impairment.

      2.6 Certificate as to Adjustments. Upon each adjustment of the Warrant
Price, the Company at its expense shall promptly compute such adjustment, and
furnish Holder with a certificate of its Controller or Chief Financial Officer
setting forth such adjustment and the facts upon which such adjustment is based.
The Company shall, upon written request, furnish Holder

                                       3
<PAGE>

a certificate setting forth the Warrant Price in effect upon the date thereof
and the series of adjustments leading to such Warrant Price.

ARTICLE 3. REPRESENTATIONS AND COVENANTS OF THE COMPANY.

      3.1 Representations and Warranties. The Company hereby represents and
warrants to the Holder as follows:

            (a) The initial Warrant Price referenced on the first page of this
warrant is not greater than the fair market value of the Shares as of the date
of this warrant.

            (b) All Shares which may be issued upon the exercise of the purchase
right represented by this warrant, and all securities, if any, issuable upon
conversion of the Shares, shall, upon issuance, be duly authorized, validly
issued, fully paid and nonassessable, and free of any liens and encumbrances
except for restrictions on transfer provided for herein or under applicable
federal and state securities laws.

            (c) The Company's capitalization table attached to this warrant is
true and complete as of the Issue Date.

      3.2 Notice of Certain Events. If the Company proposes at any time (a) to
declare any dividend or distribution upon its common stock, whether in cash,
property, stock, or other securities and whether or not a regular cash dividend;
(b) except pursuant to that certain Investor Rights Agreement, as amended,
between the Company and certain investors, to offer for subscription pro rata to
the holders of any class or series of its stock any additional shares of stock
of any class or series or other rights; (c) to effect any reclassification or
recapitalization of common stock; or (d) to merge or consolidate with or into
any other corporation, or sell, lease, license, or convey all or substantially
all of its assets, or to liquidate, dissolve or wind up, then, in connection
with each such event, the Company shall give Holder (1) at least 20 days prior
written notice of the date on which a record will be taken for such dividend,
distribution, or subscription rights (and specifying the date on which the
holders of common stock will be entitled thereto) or for determining rights to
vote, if any, in respect of the matters referred to in (a) and (b) above; and
(2) in the case of the matters referred to in (c) and (d) above at least 20 days
prior written notice of the date when the same will take place (and specifying
the date on which the holders of common stock will be entitled to exchange their
common stock for securities or other property deliverable upon the occurrence of
such event).

      3.3 Information Rights. So long as the Holder holds this warrant and/or
any of the Shares, the Company shall deliver to the Holder (a) promptly after
mailing, copies of all communiques to the shareholders of the Company, (b)
within one hundred fifty (150) days after the end of each fiscal year of the
Company, the annual audited financial statements of the Company certified by
independent public accountants of recognized standing and (c) within forty-five
(45) days after the end of each of the first three quarters of each fiscal year,
the Company's quarterly, unaudited financial statements.

      3.4 Registration Under Securities Act of 1933, as amended. The Company
agrees that the Shares or, if the Shares are convertible into common stock of
the Company, such common stock, shall be subject to the registration rights set
forth on Exhibit B.

                                       4
<PAGE>

ARTICLE 4. MISCELLANEOUS.

      4.1 Term: Notice of Expiration. This warrant is exercisable in whole or in
part, at any time and from time to time on or before the Expiration Date set
forth above. If this warrant has not been exercised prior to the Expiration
Date, this warrant shall be deemed to have been automatically exercised on the
Expiration Date by "cashless" conversion pursuant to Section 1.2.

      4.2 Legends. This warrant and the Shares (and the securities issuable,
directly or indirectly, upon conversion of the Shares, if any) shall be
imprinted with a legend in substantially the following form:

THIS SECURITY HAS NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, OR ANY
APPLICABLE STATE SECURITIES LAW, AS AMENDED, AND MAY NOT BE SOLD, PLEDGED OR
OTHERWISE TRANSFERRED WITHOUT AN EFFECTIVE REGISTRATION THEREOF UNDER SUCH ACT
AND LAWS OR PURSUANT TO RULE 144 OR AN OPINION OF COUNSEL REASONABLY
SATISFACTORY TO THE CORPORATION AND ITS COUNSEL THAT SUCH REGISTRATION IS NOT
REQUIRED.

      4.3 Compliance with Securities Laws on Transfer. The Holder acknowledges
that this warrant and the Shares issuable upon exercise of this warrant (and the
securities issuable, directly or indirectly, upon conversion of the Shares, if
any) are being acquired solely for the Holder's own and its affiliates' account
and not as nominee for any other party, and for investment and that this warrant
and the shares issuable upon exercise of this warrant (and the securities
issuable, directly or indirectly, upon conversion of the shares) may not be
transferred or assigned in whole or in part without compliance with applicable
federal and state securities laws by the transferor and the transferee
(including, without limitation, the delivery of investment representation
letters and legal opinions reasonably satisfactory to the Company). The Company
shall not require Holder to provide an opinion of counsel if the transfer is to
an affiliate of Holder or if there is no material question as to the
availability of current information as referenced in Rule 144(c), Holder
represents that it has complied with Rule 144(d) and (e) in reasonable detail,
the selling broker represents that it has complied with Rule 144(f), and the
Company is provided with a copy of Holder's notice of proposed sale.

      4.4 Transfer Procedure. Subject to the provisions of Section 4.3, Holder
may transfer all or part of this warrant or the Shares issuable upon exercise of
this warrant (or the securities issuable, directly or indirectly, upon
conversion of the Shares, if any) by giving the Company notice of the portion of
the warrant being transferred setting forth the name, address and taxpayer
identification number of the transferee and surrendering this warrant to the
Company for reissuance to the transferee(s) (and Holder, if applicable);
provided, however, that Holder may transfer all or part of this warrant to its
affiliates, including, without limitation, Comerica Incorporated, at any time
without notice to the Company, and such affiliate shall then be entitled to all
the rights of Holder under this warrant and any related agreements, and the
Company shall cooperate fully in ensuring that any stock issued upon exercise of
this warrant is issued in the name of the affiliate that exercises the warrant.
The terms and conditions of this warrant shall inure to the benefit of, and be
binding upon, the Company and the holders hereof and their respective permitted
successors and assigns.

                                       5
<PAGE>

      4.5 Notices. All notices and other communications from the Company to the
Holder, or vice versa, shall be deemed delivered and effective when given
personally or mailed by first-class registered or certified mail, postage
prepaid, at such address as may have been furnished to the Company or the
Holder, as the case may be, in writing by the Company or such Holder from time
to time. All notices to the Holder shall be addressed as follows:

                               Comerica Bank-California

                                       Attn: Warrant Administrator
                                       Technology and Life Sciences Division
                                       P.O. Box 7279
                                       San Francisco, CA 94120-7279

                               With a copy to:

                                       Comerica Bank-California
                                       Attn: Warrant Administrator
                                       Technology and Life Sciences Division
                                       5 Palo Alto Square, Suite 800
                                       3000 El Camino Real
                                       Palo Alto, CA 94306

      4.6 Waiver. This warrant and any term hereof may be changed, waived,
discharged or terminated only by an instrument in writing signed by the party
against which enforcement of such change, waiver, discharge or termination is
sought.

      4.7 Attorneys' Fees. In the event of any dispute between the parties
concerning the terms and provisions of this warrant, the party prevailing in
such dispute shall be entitled to collect from the other party all costs
incurred in such dispute, including reasonable attorneys' fees.

      4.8 Governing Law. This warrant shall be governed by and construed in
accordance with the laws of the State of California, without giving effect to
its principles regarding conflicts of law.

                                                 NXSTAGE MEDICAL, INC.

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

                                                 Name:  Jeffrey H. Burbank

                                                 Title:  President & CEO

                                       6
<PAGE>

                                   APPENDIX I

                               NOTICE OF EXERCISE

      1. The undersigned hereby elects to purchase _________ shares of _________
stock of NxStage Medical, Inc. pursuant to the terms of the attached warrant,
and tenders herewith payment of the purchase price of such shares in full.

      1. The undersigned hereby elects to convert the attached warrant into
shares in the manner specified in the warrant. This conversion is exercised with
respect to _________ of the shares covered by the warrant.

      [STRIKE PARAGRAPH THAT DOES NOT APPLY]

      2. Please issue a certificate or certificates representing said shares in
the name of the undersigned or in such other name as is specified below:

            Comerica Bank - California
            Attn:  Warrant Administrator
            Technology and Life Sciences Division
            P.O. Box 7279
            San Francisco, CA 94120-7279

            OR Registered Assignee

      3. The undersigned represents it is acquiring the shares solely for its
own account and not as a nominee for any other party and not with a view toward
the resale or distribution thereof except in compliance with applicable
securities laws.

COMERICA BANK - CALIFORNIA or Registered Assignee

_________________________
(Signature)

_________________________
(Date)

                                       1
<PAGE>

                                    EXHIBIT A

                            ANTI-DILUTION PROVISIONS

      In the event of the issuance (a "Diluting Issuance") by the Company, after
the issue Date of the warrant, of securities at a price per share less than the
Warrant Price, then the number of shares of common stock issuable upon
conversion of the Shares shall be adjusted in accordance with those provisions
(the "Provisions") of the Company's Certificate of Incorporation which apply to
Diluting Issuances.

      The Company agrees that the Provisions, as in effect on the Issue Date,
shall be deemed to remain in full force and effect during the term of the
warrant notwithstanding any subsequent amendment, waiver or termination thereof
by the Company's shareholders.

      Under no circumstances shall the aggregate Warrant Price payable by the
Holder upon exercise of the warrant increase as a result of any adjustment
arising from a Diluting Issuance.

                                       1
<PAGE>

                                    EXHIBIT B

                               REGISTRATION RIGHTS

      The common stock issuable upon conversion of the Shares, shall be deemed
"registrable securities" in accordance with the terms of the Investor Rights
Agreement dated June 30, 1999, as amended by the First Amendment to the
Investors Rights Agreement dated January 24, 2000 and the Second Amendment to
the Investors Rights Agreement dated May 24, 2001 (collectively, the
"Agreement"). By acceptance of the Warrant to which this Exhibit B is attached,
Holder shall be deemed to be a party to the Agreement.

                                       1

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