SecProbe.io

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								August 15, 2005

Mail Stop 6010

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

      Re:	NxStage Medical, Inc.
      Registration Statement on Form S-1
      Filed July 19, 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.

      Please understand that the purpose of our review process is
to
assist you in your compliance with the applicable disclosure
requirements and to enhance the overall disclosure in your filing.
We look forward to working with you in these respects.  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.

Form S-1

1. Please confirm that any preliminary prospectus you circulate
will
include all non-Rule 430A information.  This includes the price
range
and related information based on a bona fide estimate of the
public
offering within that range.

Prospectus Summary, page 1
2. In an appropriate place in your document, please identify your
subsidiary and briefly describe the nature of its operations.
Disclose the extent of your ownership interest in the subsidiary.
3. If true, please revise to clarify that although your system
allows
more than the typical three treatments per week, a customer`s
decision to use your product may be influenced by the fact that
current rules of the Centers for Medicare and Medicaid Services
limit, absent medical justification, the number of treatments
Medicare will reimburse to three per week.  Explain that the
adoption
of your system may be slowed if Medicare contractors are unwilling
to
pay for the additional treatments.
4. Please provide independent support for the statistics cited
throughout the Summary.
5. Please clarify whether the 141 patients mentioned in the fifth
paragraph are home users.  Also clarify the number of clinics
through
which you currently derive revenue; if these clinics are
concentrated
in one or a few geographic areas, please disclose those areas.

Summary Consolidated Financial Data, page 6

6. Please revise the pro forma balance sheet data to separately
reflect the impact of the offering.  We note that the current
presentation combines the impact of the offering and the assumed
conversion of your preferred stock.  Please similarly revise your
capitalization table on page 27.

Risk Factors, page 8

Ownership of dialysis clinics is highly concentrated..., page 11

7. With a view toward disclosure, please tell us the portion of
your
business derived from clinics that are subject to pending
acquisitions by competitors.

We are subject to the risk of costly and damaging product
liability
claims..., page 12

8. Please revise to explain the circumstances leading to the two
voluntary recalls and the product withdrawal.  Disclose when these
occurred and describe the reasons for the events.

We currently have obligations..., page 16

9. Please clarify how your operations could be materially affected
by
privacy laws. Likewise, in the next risk factor, please clarify
what
aspect of your operations put you at material risk of violating
the
anti-kickback laws.

Use of Proceeds, page 25

10. Please disclose the portion of the proceeds needed for each
disclosed use based on currently available information.

Dilution, page 28

11. Disclose the material effect on the information presented in
this
section of exercise of options, warrants and other obligations to
issue securities.

Management`s Discussion and Analysis of Financial Condition and
Results of Operations, page 32

12. Since changes in your methodologies and assumptions could have
a
material impact upon your valuation of stock options in the
financial
statements, we believe that the following disclosures in MD&A
would
be helpful to an investor:

* The aggregate intrinsic value of all outstanding options based
on
the midpoint of the estimated IPO price range.

* The significant factors, assumptions and methodologies used in
determining fair value for those options granted during the twelve
months prior to the date of the most recent balance sheet.

* A discussion of each significant factor contributing to the
difference between the fair value as of the date of grant and the
estimated IPO price for options granted during the twelve months
prior to the date of the most recent balance sheet.

* Disclose the valuation method used and the reasons why you
choose
that method.
13. Please revise to address briefly the nature of the third-party
supply issue that caused you to suspend sales activity in 2002.
Disclose how this issue was resolved and whether you believe a
similar issue represents a material risk to your business today.
Overview, page 32
14. Please clarify what an IDE study is and how you could commence
commercial production before completion of the study and FDA
review.
Revenues, page 33
15. Please expand your disclosure to describe the material terms
of
the monthly rental agreements for the rental of your System One
for
use in patient homes.

Research and Development, page 36

16. Clarify why you needed to design a new dialyzer.

Liquidity and Capital Resources, page 37

17. Please reconcile your notes payable of approximately $6
million
in your table of contractual commitments to your long-term debt of
approximately $4.5 million as shown in your December 31, 2004
balance
sheet on page F-3.

Business, page 44

Overview, page 44
18. Revise to describe in greater detail the duration, nature and
degree of preparation a patient would need to receive to operate
your
system.  Disclose any costs to the patient.
19. Please tell us who conducted the surveys mentioned in the
second
paragraph, whether the surveys were commissioned by you, and how
you
ensured that they were scientific.

Market Opportunity, page 45
20. Please provide us with independent support for the industry
data
cited throughout this section.
21. Tell us how you compensate those who provide data for or
endorse
your products.  Tell us about those individuals` interests in your
company.
22. Please tell us, and provide, the evidence demonstrating that
the
risk of infection is higher among patients receiving in-center
hemodialysis than among patients using home hemodialysis.

Designed for safety, page 51

23. Please tell us your experience with operator error and other
safety concerns. Compare your rate of negative outcomes with
competitive treatments.

Intellectual Property, page 55
24. Please revise to disclose the duration of the patents listed.

Competition, page 56

25. Describe your competitive position.  For example, what is your
position relative to your competitors based on the factors in the
first bullet list on page 56?  How does your product compare based
on
effectiveness of the treatment?

Manufacturing, page 57

26. Disclose the material terms of the agreements with your
suppliers, including duration, termination provisions, and
material
obligations of the parties.  Likewise, in an appropriate section
of
your document, disclose the material terms of the business
associate
agreements mentioned on page 16.

Food and Drug Administration, page 59

27. Disclose the dates of your material FDA clearances and the
scope
of those clearances.

Scientific Advisory Board, page 67

28. If the board serves a management role, please provide complete
disclosure as required for other directors, such as the disclosure
by
Regulation S-K Item 401.  Otherwise, relocate the disclosure about
the board to an appropriate section of your prospectus other than
the
"Management" section.  Also ensure that the extent of the board`s
role is clear.   For example:
* How often do they meet?
* What is the extent of their obligations to you?
* What are your obligations to them?
* Do they have an equity interest?
* Are they the source of performance data regarding your product?

Option Exercises, page 72

29. Please disclose the year-end value of common stock used for
the
calculations in this table.

Principal Stockholders, page 76
30. Identify the individuals who beneficially own the shares held
by
the entities named in the table.

Related-Party Transactions, page 80

31. Please provide disclosure for the last three fiscal years.
See
Instruction 2 to Regulation S-K Item 404.  We note, for example,
the
omission of a discussion of the 2002 revenue from Medisystems
mentioned on page F-20.
32. Disclose the terms and effects of the stockholders agreement
mentioned at the top of page 68. File the agreement as an exhibit.

Underwriting, page 95

33. Identify any members of the underwriting syndicate that will
engage in any electronic offer, sale or distribution of the shares
and describe their procedures to us.  If you become aware of any
additional members of the underwriting syndicate that may engage
in
electronic offers, sales or distributions after you respond to
this
comment, promptly supplement your response to identify those
members
and provide us with a description of their procedures.

	Also, in your discussion of your procedures, tell us how your
procedures ensure 	that the distribution complies with Section 5
of
the Securities Act.  In particular:

* the communications used;
* the availability of the preliminary prospectus;
* the manner of conducting the distribution and sale, such as the
use
of indications of interest or conditional offers; and
* the funding of an account and payment of the purchase price.

Finally, tell us whether you or the underwriters have any
arrangements with a third party to host or access your preliminary
prospectus on the Internet.  If so, identify the party and the
website, describe the material terms of your agreement, and
provide
us with a copy of any written agreement.  Provide us also with
copies
of all information concerning your issuer or prospectus that has
appeared on their website.  Again, if you subsequently enter into
any
such arrangements, promptly supplement your response.

	We may have further comment.

Reserved Shares, page 96

34. We note that the underwriters have reserved a certain number
of
shares to be sold in this offering to directors, officers,
employees,
business associates and related persons.  Please provide us with
the
following information:
* Please tell us the circumstances leading to the allotment of
shares
being 	reserved, including the timing and content of all
communications with 	potential participants;
* Provide details on how the reserved share plan is being
conducted,
including 	how the prospective participants and the number of
reserved shares will be 	determined;
* Tell us whether purchasers are required to establish accounts
with
the 	underwriters before the effective time, and, if so, whether
any
funds are 	deposited into any newly established brokerage
accounts before the effective 	date of the 	registration
statement;
* Tell us whether the procedures for the reserved shares differ
from
the 	procedures	for the general offering to the public;
* We note that some of the persons who may purchase the shares are
also 	within the categories of persons described in Item 404
of
Regulation S-K.  	If applicable, please add the disclosure
required
by Item 404 in an 	appropriate place in your document; and
* Provide a copy of all written material used in connection with
the
reserved 	share program and analyze how that material is
consistent
with Rule 134.

Price Stabilization, page 97

35. Please expand your disclosure to address the issues mentioned
in
Section VIII.A.3 of the Division of Corporation Finance`s November
14, 2000 outline of Current Issues and Rulemaking Projects
available
on our web site at http://www.sec.gov/worddocs/cfcr112k.doc.

Other Relationships, page 98

36. Please provide more specific disclosure regarding the nature
of
the relationships and the compensation paid.

Consolidated Financial Statements, page F-1

37. Please update the financial statements and related information
in
your filing to comply with Rule 3-12 of Regulation S-X.

Consolidated Statements of Cash Flows, page F-6

38. Please tell us how you determined the amount of the investment
in
field equipment for each period presented and whether this amount
represents a non-cash transfer from inventory.

Note 1. Operations, page F-7

39. Please tell us why you refer the reader to Note 7 instead of
Note
6 where the terms of conversion are disclosed.  Please tell us and
disclose the number of shares that may be issuable upon conversion
due to the contingent conversion feature of the preferred stock.

Note 2. Summary of Significant Accounting Policies, page F-8

Revenue Recognition, page F-10

40. Please tell us the significant terms of your various rental
agreements and how you evaluated those terms in determining the
type
of lease arrangement and the related accounting for and
measurement
of the leased asset and revenue recognition.

41. Please tell us and disclose the nature of the services you
provide and how you recognize and measure revenue related to these
services, including clinical training.  Cite the accounting
literature upon which you relied and explain how you applied that
literature to your situation.

42. Please tell us and disclose how you account for the costs
related
to the equipment under the disposable-based program and tell us
why.

43. With respect to your multiple element arrangements, please
tell
us and disclose, the contract elements permitting separate revenue
recognition, describing how they are distinguished.  Please also
explain how you allocate contract revenue among the elements and
your
method of revenue recognition for each element.

Distribution Expenses, page F-11

44. Please tell us why you only charge distribution costs to
operations if they are not charged to customers.  Tell us and
disclose how you accounted for shipping and handling costs billed
to
customers prior to 2004 and tell us how your accounting complies
with
EITF 00-10.

Note 5. Financing Arrangements, page F-15

Debt, page F-15

45. Please tell us how you are accounting for the $650,000 balloon
payment and why.  Clarify whether this amount represents
additional
interest on the loan.  Please also clarify the method you are
using
to amortize the value of the warrants to interest expense.

Future Maturities, page F-16

46. Please reconcile the total amount in the table of future
maturities with your balance sheet on page F-3.

Conversion, page F-17

47. Please tell us and disclose how you are accounting for the
contingent beneficial conversion feature of the Series F preferred
stock in the event of an initial public offering and tell us why.

Note 7. Stock Options, page F-18

48. For options granted or issued during the 12 months prior to
the
date of the most recent balance sheet, please provide us with
details
of the significant factors contributing to the difference between
the
underlying fair value of those issuances and the price at which
you
sold your preferred stock during those periods.

49. Please provide the estimated IPO price or range when
available.
If the difference between the IPO price and the fair value used to
value stock, options and warrants granted or issued during the 12
months prior to the date of the most recent balance sheet is
significant please provide us with details of the significant
factors
contributing to the difference.  Please note that we will defer
our
final evaluation of stoc