Exhibit 31.1
 CERTIFICATION PURSUANT TO RULE 13A-14(A)/15D-14(A),
AS ADOPTED PURSUANT TO
SECTION 302 OF THE SARBANES-OXLEY ACT OF 2002
  1.
    
    I have reviewed this Quarterly Report on Form 10-Q/A of
NxStage Medical, Inc. for the period ended June 30, 2011
(this report);2.
    
    Based on my knowledge, this report does not contain any
untrue statement of a material fact or omit to state a
material fact necessary to make the statements made, in
light of the circumstances under which such statements were
made, not misleading with respect to the period covered by
this report;3.
    
    Based on my knowledge, the financial statements, and other
financial information included in this report, fairly
present in all material respects the financial condition,
results of operations and cash flows of the registrant as
of, and for, the periods presented in this report;4.
    
    The registrants other certifying officer and I are
responsible for establishing and maintaining disclosure
controls and procedures (as defined in Exchange Act Rules
13a-15(e) and 15d-15(e)) and internal control over financial
reporting (as defined in Exchange Act Rules 13a-15(f) and
15d-15(f)) for the registrant and have:
 a)
    
    Designed such disclosure controls and
procedures, or caused such disclosure
controls and procedures to be designed
under our supervision, to ensure that
material information relating to the
registrant, including its consolidated
subsidiaries, is made known to us by
others within those entities,
particularly during the period in which
this report is being prepared;b)
    
    Designed such internal control over
financial reporting, or caused such
internal control over financial reporting
to be designed under our supervision, to
provide reasonable assurance regarding
the reliability of financial reporting
and the preparation of financial
statements for external purposes in
accordance with generally accepted
accounting principles;c)
    
    Evaluated the effectiveness of the
registrants disclosure controls and
procedures and presented in this report
our conclusions about the effectiveness
of the disclosure controls and
procedures, as of the end of the period
covered by this report based on such
evaluation; andd)
    
    Disclosed in this report any change in
the registrants internal control over
financial reporting that occurred during
the registrants most recent fiscal
quarter (the registrants fourth fiscal
quarter in the case of an annual report)
that has materially affected, or is
reasonably likely to materially affect,
the registrants internal control over
financial reporting; and
 5.
    
    The registrants other certifying officer and I have
disclosed, based on our most recent evaluation of internal
control over financial reporting, to the registrants
auditors and the audit committee of the registrants board
of directors (or persons performing the equivalent
functions):
 a)
    
    All significant deficiencies and material
weaknesses in the design or operation of
internal control over financial reporting
which are reasonably likely to adversely
affect the registrants ability to
record, process, summarize and report
financial information; andb)
    
    Any fraud, whether or not material, that
involves management or other employees
who have a significant role in the
registrants internal controls over
financial reporting.
 President and Chief Executive Officer