Sec Form 4 Filing - ATLAS VENTURE FUND III L P @ NxStage Medical, Inc. - 2006-05-26

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person ATLAS VENTURE FUND III L P 2. Issuer Name and Ticker or Trading Symbol NxStage Medical, Inc. [ NXTM] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) 890 WINTER STREET, SUITE 320 3. Date of Earliest Transaction (MM/DD/YY) 05/26/2006 (Street) WALTHAM, MA02451 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) _____ Form filed by One Reporting Person __ X __ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 05/26/2006 X 10,434 ( 1 ) A $ 7.13 691,068 ( 2 ) I See Footnote ( 2 ) Common Stock 05/26/2006 S 6,795 ( 1 ) D $ 10.95 691,068 ( 2 ) I See Footnote ( 2 )

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares Warrants to Purchase 10,434 Equity Securities $ 7.13 05/26/2006 X 10,434 ( 1 ) 11/21/2005 05/31/2006 Common Stock 10,434 ( 1 ) $ 0 0 D

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other ATLAS VENTURE FUND III L P 890 WINTER STREET, SUITE 320 WALTHAM, MA02451 X ATLAS VENTURE ENTREPRENEURS FUND III L P 890 WINTER STREET, SUITE 320 WALTHAM, MA02451 X ATLAS VENTURE ASSOCIATES III L P 890 WINTER STREET, SUITE 320 WALTHAM, MA02451 X Signatures /s/ Jeanne Larkin Henrey 05/31/2006 Signature of Reporting Person Date

Explanation of Responses:

( 1 )See "Notes to Footnote 1" on Exhibit 99.2. ( 2 )Following the reported transaction, Atlas Venture Fund III, L.P. holds 676,366 shares and Atlas Venture Enterpreneurs' Fund III, L.P. holds 14,702 shares of the Issure's common stock. The reporting persons disclaim beneficial ownership of such shares except to the extent of their pecuniary interest therein.

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).

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