RECITALS

                                                                    Exhibit 10.4

                AMENDMENT TO NON-QUALIFIED STOCK OPTION AGREEMENT

      THIS AMENDMENT TO NON-QUALIFIED STOCK OPTION AGREEMENT (this "Amendment")
is entered into as of the 24th day of March 2006, by and among NXSTAGE MEDICAL,
INC., a Delaware corporation (the "Company"), and XXXXXX X. XXXXXX ("Optionee").

                                    RECITALS

      WHEREAS, the Company and Optionee desire to amend certain terms of the
certain Non-Qualified Stock Option Agreement between the Company and Optionee
dated as of October 25, 2004 (the "Option Agreement");

      NOW THEREFORE, in consideration of the mutual promises, representations,
warranties, covenants and conditions set forth herein, the parties hereby agree
as follows:

1. AMENDMENT TO OPTION AGREEMENT. In accordance with Section 12 of the Option
Agreement, the Company and Optionee hereby agree to amend the Option Agreement
as follows:

      a. The Option Exercise Price/Share is hereby increased from $3.00 to $4.00
(or from $4.10, after giving effect to the reverse stock split affected by the
Company in connection with its initial public offering closed on November 1,
2005, to $5.47).

      b. The remainder of the Option as not amended hereby, remains unchanged
and in full force and effect.

      c. In consideration for such amendment, the Company hereby agrees to (i)
pay Optionee $115,750 in cash on or promptly after January 1, 2007; provided
Optionee continues to be employed by the Company as of such date, and (ii) make
a grant of 13,027 shares of restricted stock to Optionee, 2991 of which shall
vest as of January 1, 2007, provided Optionee continues to be employed by the
Company as of such date, and the remainder of which shall vest in approximately
equal amounts through October 24, 2008, again, provided Optionee continues to be
employed by the Company as of each such vesting date.



2. MISCELLANEOUS

      2.1 GOVERNING LAW. This Amendment is governed by and construed under the
laws of the Commonwealth of Massachusetts as applied to agreements among
Massachusetts residents entered into and to be performed entirely within
Massachusetts.

      2.2 COUNTERPARTS. This Amendment may be executed in any number of
counterparts, each of which is an original, and all of which together
constitutes one instrument.

      IN WITNESS WHEREOF, the parties hereto have executed this Amendment to the
Option Agreement as of the date first set forth above.

                                               COMPANY

                                               NXSTAGE MEDICAL, INC.
                                               000 XXXXX XXXXX XXXXXX, 0xx XXXXX
                                               XXXXXXXX, XX 00000

                                               By:______________________________
                                               Name:
                                               Title:

                                               OPTIONEE:

                                               _________________________________
                                               Xxxxxx X. Xxxxxx