NxStage Medical 8-K/A Completion of Acquisition or Disposition of Assets

Exhibit 23.1

CONSENT OF INDEPENDENT PUBLIC ACCOUNTING FIRM

We hereby consent to the incorporation by reference in the Registration Statements of NxStage Medical, Inc. on Forms S-8 (File Nos. 333-141804 and 333-130241) and Form S-3 (File No. 333-141808), of our report dated July 13, 2007 on the financial statements of the Medisystems Group Companies which is incorporated by reference in this Current Report on Form 8-K/A.

/s/ GRANT THORNTON LLP

Seattle, Washington
December 14, 2007