SecProbe.io

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								October 18, 2005

Mail Stop 6010

Jeffrey H. Burbank
President and Chief Executive Officer
NxStage Medical, Inc.
439 South Union Street, 5th Floor
Lawrence, Massachusetts 01843

      Re:	NxStage Medical, Inc.
      Amendment No. 2 to Registration Statement on Form S-1
      Filed October 7, 2005
		File No. 333-126711

Dear Mr. Burbank:

      We have reviewed your filing and have the following
comments.
Where indicated, we think you should revise your document in
response
to these comments.  If you disagree, we will consider your
explanation as to why our comment is inapplicable or a revision is
unnecessary.  Please be as detailed as necessary in your
explanation.
In some of our comments, we may ask you to provide us with
information so we may better understand your disclosure.  After
reviewing this information, we may raise additional comments.

      We welcome any questions you may have about our comments or
any
other aspect of our review.  Feel free to call us at the telephone
numbers listed at the end of this letter.

Amendment No. 2 to Form S-1

Designed for safety, page 57

1. Please balance the disclosure in this section with information
about your experience with operator error.

Employment Agreements, page 79

2. Please file as exhibits the employment agreements described in
this section.

Underwriting, page 108

Electronic Distribution, page 111
3. It appears from your added disclosure under this caption that,
in
addition to Merrill Lynch, one or more of the underwriters in the
offering will make available a prospectus in electronic format on
their websites.  Please identify all members of the underwriting
syndicate that will engage in any electronic offer, sale or
distribution of the shares and describe their procedures to us.
Refer to comment 33 in our letter dated August 15, 2005.

Part II

Exhibit 5.1 Opinion of Wilmer Cutler Pickering Hale and Dorr LLP
4. Please provide confirmation from counsel that it concurs with
our
understanding that the reference and limitation in its opinion to
the
General Corporation Law of the State of Delaware includes the
statutory provisions and also all applicable provisions of the
Delaware Constitution and reported judicial decisions interpreting
these laws.
5. We note counsel`s consent to being named in the prospectus
under
the caption "Validity of Common Stock."  It appears, however, that
counsel`s name appears in the prospectus under a different
caption.
Please revise accordingly.

      As appropriate, please amend your registration statement in
response to these comments.  You may wish to provide us with
marked
copies of the amendment to expedite our review.  Please furnish a
cover letter with your amendment that keys your responses to our
comments and provides any requested information.  Detailed cover
letters greatly facilitate our review.  Please understand that we
may
have additional comments after reviewing your amendment and
responses
to our comments.

      We urge all persons who are responsible for the accuracy and
adequacy of the disclosure in the filing to be certain that the
filing includes all information required under the Securities Act
of
1933 and that they have provided all information investors require
for an informed investment decision.  Since the company and its
management are in possession of all facts relating to a company`s
disclosure, they are responsible for the accuracy and adequacy of
the
disclosures they have made.

      We will consider a written request for acceleration of the
effective date of the registration statement as confirmation of
the
fact that those requesting acceleration are aware of their
respective
responsibilities under the Securities Act of 1933 and the
Securities
Exchange Act of 1934 as they relate to the proposed public
offering
of the securities specified in the above registration statement.
We
will act on the request and, pursuant to delegated authority,
grant
acceleration of the effective date.

      We direct your attention to Rules 460 and 461 regarding
requesting acceleration of a registration statement.  Please allow
adequate time after the filing of any amendment for further review
before submitting a request for acceleration.  Please provide this
request at least two business days in advance of the requested
effective date.

      You may contact Julie Sherman at (202) 551-3640 or Kaitlin
Tillan at (202) 551-3604 if you have questions regarding comments
on
the financial statements and related matters.  Please contact
Eduardo
Aleman at (202) 551-3646 or me at (202) 551-3617 with any other
questions.

      					Sincerely,

      					Russell Mancuso
      Branch Chief

cc(via facsimile):	Mark. G. Borden, Esq.

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Jeffrey H. Burbank
NxStage Medical, Inc.
October 18, 2005
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