STOCK PURCHASE AGREEMENT

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Exhibit 10.31



STOCK PURCHASE AGREEMENT


This Stock Purchase Agreement, dated as of February 7, 2007, is made
and entered into by and between NxStage Medical, Inc., a Delaware corporation
(the "Company"), and DaVita Inc., a Delaware corporation (the "Purchaser"). The
Company and the Purchaser are collectively referred to as the "parties."

WHEREAS, concurrently herewith, the parties have entered into a
National Service Provider Agreement (the "Service Provider Agreement"); and

WHEREAS, in connection with, and as a condition of the parties entering
into the Service Provider Agreement, the Company desires to issue and sell, and
the Purchaser desires to purchase, shares of the Company's common stock, $0.001
par value per share (the "Common Stock"); and

NOW THEREFORE, in consideration of the representations, warranties and
covenants herein contained, and other good and valuable consideration, the
receipt and adequacy of which is hereby acknowledged, the Company and the
Purchaser agree as follows:

1. Sale of Shares. Subject to the terms and conditions of this
Agreement, at the Closing (as defined in Section 2 hereof) the Purchaser agrees
to purchase, and the Company agrees to sell and issue to the Purchaser, that
number of shares ("Shares") equal to the quotient of $20,000,000 divided by the
higher of (i) $10.00 and (ii) the average of the closing sales prices per share
of the Common Stock on the NASDAQ Global Market for the twenty (20) consecutive
trading days immediately preceding the Effective Date (as defined in the Service
Provider Agreement").

2. The Closing.

2.1 Closing. The closing (the "Closing") of the sale and
purchase of the Shares under this Agreement will take place at the offices of
WilmerHale, 60 State Street, Boston, Massachusetts at 10:00 a.m. on February 7,
2007, or at such other time, date and place as are mutually agreeable to the
Company and the Purchaser. The date of the Closing is hereinafter referred to as
the "Closing Date."

2.2 Delivery of Shares; Payment of Purchase Price. At the
Closing, the Company will deliver to the Purchaser a certificate registered in
the name of the Purchaser representing the Shares against payment therefor by
the Purchaser to the Company by wire transfer of $20,000,000.

3. Representations of the Company. The Company hereby makes the
following representations and warranties to the Purchaser:

3.1 Charter and By-laws. The Company has made available
to the Purchaser true, correct and complete copies of the Company's Restated
Certificate of Incorporation and By-laws, each as in effect on the date hereof.


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3.2 Valid Issuance of Common Stock. When issued, sold and
delivered in accordance with the terms of this Agreement for the consideration
expressed in this Agreement, the Shares will be duly authorized, validly issued,
fully paid, non-assessable and free of restrictions on transfer other than
restrictions imposed or created under this Agreement, by applicable law or by
the Purchaser and, based in part upon the representations of the Purchaser in
this Agreement, will be issued in compliance in all material respects with
applicable U.S. federal securities laws.

3.3 Capitalization. The authorized capital stock of the
Company, as of December 31, 2006, consists of 100,000,000 shares of Common
Stock, of which 28,806,543 shares are issued and outstanding and 5,000,000
shares of blank check Preferred Stock, $0.001 par value per share, none of which
have been designated. Options to purchase an aggregate of 3,068,430 shares of
Common Stock were outstanding as of December 31, 2006. Except as set forth
herein and in the Company SEC Documents (as defined below), there are no
outstanding, rights (including conversion rights or preemptive rights and rights
of first refusal), options, warrants, instruments, or other agreements or
instruments of any kind to which the Company is a party for the purchase or
acquisition from the Company of any securities of the Company.

3.4 Financial Statements and SEC Filings. Since January
1, 2006, the Company has filed all required reports, schedules, forms, financial
statements and other documents (including exhibits and all other information
incorporated therein) with the Securities and Exchange Commission (the "Company
SEC Documents"). As of their respective dates, the Company SEC Documents
complied as to form in all material respects with the requirements of the
Securities Act of 1933, as amended (the "Securities Act") or the Securities
Exchange Act of 1934, as amended (the "Exchange Act"), as the case may be, and
the rules and regulations of the SEC promulgated thereunder applicable to such
Company SEC Documents. At the time they were filed with the SEC, the Company SEC
Documents did not contain any untrue statement of a material fact or omit to
state a material fact required to be stated therein or necessary in order to
make the statements therein, in the light of the circumstances under which they
were made, not misleading. As of their respective dates, the financial
statements of the Company included in the Company SEC Documents, as amended,
complied as to form in all material respects with applicable accounting
requirements and the published rules and regulations of the SEC with respect
thereto. Such financial statements have been prepared in accordance with
generally accepted accounting principles, consistently applied, during the
periods involved (except (i) as may be otherwise indicated in such financial
statements or the notes thereto, or (ii) in the case of unaudited interim
statements, as permitted by the SEC on Form 10-Q) and fairly presented in all
material respects the financial position of the Company as of the dates thereof
and the results of its operations and cash flows for the periods then ended
(subject, in the case of unaudited statements, to normal year-end audit
adjustments).

3.5 NASDAQ Listing. As of the date hereof, the Common
Stock is listed on the NASDAQ Global Market and the Company has not received any
notification that the NASDAQ Global Market is contemplating terminating such
listing.

3.6 No Material Adverse Change. Since the filing of the
Company's most recent Annual Report on Form 10-K or Quarterly Report on Form
10-Q, as the case may be, except as described or referred to in the Company SEC
Documents, there has not been any



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material adverse change in the assets, business, properties, financial condition
or results of operations of the Company.

3.7 Corporate Power/Authorization. The Company has all
requisite corporate power and authority and has taken all necessary corporate
action to execute and deliver this Agreement, to issue the Shares and to carry
out and perform its obligations hereunder. The execution, delivery and
performance of this Agreement constitutes the valid and binding obligations of
the Company, enforceable, in accordance with its terms.

3.8 Form S-3 Eligibility. As of the date hereof, the
Company is eligible to register the Shares for resale by the Purchaser using
Form S-3 promulgated under the 1933 Act, as such form is in effect on the date
hereof.

3.9 No General Solicitation. Neither the Company nor any
person acting on its behalf have engaged in any form of general solicitation or
general advertising (within the meaning of Regulation D) in connection with the
offer or sale of the Shares.

3.10 Brokers' Fees. The Company has no liability or
obligations to pay any fees or commissions to any broker, finder or agent with
respect to the transactions contemplated by this Agreement.

3.11 No Integrated Offering. Neither the Company, any
subsidiary of the Company, nor any person acting on its behalf has, directly or
indirectly, made any offers or sales of any security or solicited any offers to
buy any security, under circumstances that would require registration of any of
the Shares under the Securities Act or cause this offering of the Shares to be
integrated with prior offerings by the Company for purposes of the Securities
Act or any applicable stockholder approval provisions, including, without
limitation, under the rules and regulations of any exchange or automated
quotation system on which any of the securities of Company are listed or
designated. None of Company, any subsidiary of the Company or any Person acting
on their behalf will take any action or steps referred to in the preceding
sentence that would require registration of any of the Shares under the
Securities Act or cause the offering of the Shares to be integrated with other
offerings.

3.12 Sarbanes-Oxley Act. The Company is in material
compliance with applicable requirements of the Sarbanes-Oxley Act of 2002 that
are effective as of the date hereof, and applicable rules and regulations
promulgated by the SEC thereunder that are effective as of the date hereof.

3.13 Absence of Litigation. There is no action, suit or
proceeding, or governmental inquiry or investigation, pending, or, to the best
of the Company's knowledge, any basis therefore or threat thereof, against the
Company or any subsidiary of the Company, which would be reasonably likely to
have a material adverse effect on the assets, business, properties, financial
condition or results of operations of the Company or any subsidiary of the
Company (a "Material Adverse Effect").

3.14 Intellectual Property Rights. The Company and its
subsidiaries own or possess adequate rights or licenses to use all trademarks,
trade names, service marks, service mark registrations, service names, patents,
patent rights, copyrights, inventions, licenses,



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approvals, governmental authorizations, trade secrets and other intellectual
property rights ("Intellectual Property Rights") necessary to conduct their
respective businesses, in all material respects, as now conducted. The Company
has no knowledge of any infringement by the Company or any Company subsidiaries
of Intellectual Property Rights of others, which would be reasonably likely to
result in a Material Adverse Effect. There is no claim, action or proceeding
being made or brought, or to the knowledge of the Company, being threatened,
against the Company or any Company subsidiary regarding its Intellectual
Property Rights, which would be reasonably likely to result in a Material
Adverse Effect. The Company is unaware of any facts or circumstances, which
would be reasonably likely to give rise to any of the foregoing infringements or
claims, actions or proceedings.

3.15 Internal Accounting and Disclosure Controls. The
Company maintains accurate books and records reflecting its assets and
liabilities and maintains proper and adequate control over financial reporting
to provide reasonable assurance that (i) transactions are executed in accordance
with management's general or specific authorization, (ii) transactions are
recorded as necessary to permit preparation of the financial statements of the
Company and to maintain accountability for the assets of the Company, (iii)
access to the assets of the Company is permitted only in accordance with
management's general or specific authorization, (iv) the reporting of the assets
of the Company is compared with the existing assets of the Company at reasonable
intervals and (v) accounts, notes and other receivables and inventory were
recorded accurately, and proper and adequate procedures are implemented to
effect the collection thereof on a current and timely basis. The Company
maintains disclosure controls and procedures (as such term is defined in Rule
13a-15 under the 1934 Act) that are effective in ensuring that information
required to be disclosed by the Company in the reports that it files or submits
under the Exchange Act is recorded, processed, summarized and reported, within
the time periods specified in the rules and forms of the SEC, including, without
limitation, controls and procedures designed to ensure that information required
to be disclosed by Company in the reports that it files or submits under the
Exchange Act is accumulated and communicated to Company's management, including
its principal executive officer