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UNITED STATES SECURITIES AND EXCHANGE COMMISSION
 Washington, DC 20549
 FORM 10-Q/A
 (Amendment No. 1)
  þ
    
    QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934
 For the quarterly period ended June 30, 2011
 OR
 o
    
    TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934
 For the transition period from  to
 Commission file number: 000-51567
 NxStage Medical, Inc.
 (Exact Name of Registrant as Specified in Its Charter)
 Delaware(State or Other Jurisdiction of Incorporation or 
Organization)
    
    04-3454702(I.R.S. Employer Identification No.)439 S. Union St., 5thFloor, Lawrence, MA(Address of Principal Executive Offices)
    
    01843(Zip Code)
 (978) 687-4700  Registrants Telephone Number, Including Area Code)
(Former Name, Former Address, and Former Fiscal year, If Changed Since Last Report)
 þ o
 þ o
  Large accelerated filer o
    
    Accelerated filer þ
    
    Non-accelerated filer o
    
    Smaller reporting company o(Do not check if a smaller reporting company)
 o þ




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TABLE OF CONTENTS  PART II  OTHER INFORMATIONItem 6. ExhibitsSIGNATURESINDEX TO EXHIBITSEX-10.33EX-31.1EX-31.2    Table of Contents  EXPLANATORY NOTE
   
 PART II  OTHER INFORMATION
 
 Item 6. Exhibits
  ExhibitNumber**10.32
    
    Employment Agreement, dated April 25, 2011, between Registrant and Michael Miller,
Jr. (incorporated by reference to Exhibit 10.32 to the Companys Quarterly Report
on Form 10-Q filed with the Securities and Exchange Commission on May 4, 2011).*10.33
    
    Lease, dated as of June 22, 2011, by and between Registrant and 350 Riverwalk, LLC.**10.34
    
    Amendment No. 3 to 2005 Stock Incentive Plan of NxStage Medical, Inc.*31.1
    
    Certification of Chief Executive Officer pursuant to Exchange Act Rules 13a-14(a)
or 15d-14(a), as adopted pursuant to Section 302 of Sarbanes-Oxley Act of 2002.*31.2
    
    Certification of Chief Financial Officer pursuant to Exchange Act Rules 13a-14(a)
or 15d-14(a), as adopted pursuant to Section 302 of Sarbanes-Oxley Act of 2002.**32.1
    
    Certification of Chief Executive Officer pursuant to Exchange Act Rules 13a-14(b)
or 15d-14(b) and 18 U.S.C. Section 1350, as adopted pursuant to Section 906 of
Sarbanes-Oxley Act of 2002.**32.2
    
    Certification of Chief Financial Officer pursuant to Exchange Act Rules 13a-14(b)
or 15d-14(b) and 18 U.S.C. Section 1350, as adopted pursuant to Section 906 of
Sarbanes-Oxley Act of 2002.
 
 *
    
    Filed herewith.**
    
    Previously filed as an exhibit to the Companys Form 10-Q for the quarterly period ended June
30, 2011 filed with the Securities and Exchange Commission on August 3, 2011.
    
    Confidential treatment requested as to certain portions, which portions are omitted and filed
separately with the Securities and Exchange Commission. 2




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##### Table of Contents



SIGNATURES
  NXSTAGE MEDICAL, INC.By:
    /s/ Robert S. BrownRobert S. BrownChief Financial Officer
(Duly authorized officer and principal financial and accounting officer)  3




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##### Table of Contents



INDEX TO EXHIBITS
 ExhibitNumber**10.32
    
    Employment Agreement, dated April 25, 2011, between Registrant and Michael Miller,
Jr. (incorporated by reference to Exhibit 10.32 to the Companys Quarterly Report
on Form 10-Q filed with the Securities and Exchange Commission on May 4, 2011).*10.33
    
    Lease, dated as of June 22, 2011, by and between Registrant and 350 Riverwalk, LLC.**10.34
    
    Amendment No. 3 to 2005 Stock Incentive Plan of NxStage Medical, Inc.*31.1
    
    Certification of Chief Executive Officer pursuant to Exchange Act Rules 13a-14(a)
or 15d-14(a), as adopted pursuant to Section 302 of Sarbanes-Oxley Act of 2002.*31.2
    
    Certification of Chief Financial Officer pursuant to Exchange Act Rules 13a-14(a)
or 15d-14(a), as adopted pursuant to Section 302 of Sarbanes-Oxley Act of 2002.**32.1
    
    Certification of Chief Executive Officer pursuant to Exchange Act Rules 13a-14(b)
or 15d-14(b) and 18 U.S.C. Section 1350, as adopted pursuant to Section 906 of
Sarbanes-Oxley Act of 2002.**32.2
    
    Certification of Chief Financial Officer pursuant to Exchange Act Rules 13a-14(b)
or 15d-14(b) and 18 U.S.C. Section 1350, as adopted pursuant to Section 906 of
Sarbanes-Oxley Act of 2002.
 
 *
    
    Filed herewith.**
    
    Previously filed as an exhibit to the Companys Form 10-Q for the quarterly period ended June
30, 2011 filed with the Securities and Exchange Commission on August 3, 2011.
    
    Confidential treatment requested as to certain portions, which portions are omitted and filed
separately with the Securities and Exchange Commission. 4