Document:

<PAGE>
                                                                     Exhibit 4.1

COMMON STOCK                                                        COMMON STOCK

                          [NXSTAGE MEDICAL, INC. LOGO]

NUMBER                                                                 SHARES
  NX

  SEE REVERSE FOR           INCORPORATED UNDER THE LAWS        CUSIP 67072V 10 3
CERTAIN DEFINITIONS          OF THE STATE OF DELAWARE

THIS CERTIFIES THAT

is the owner of

            FULLY PAID AND NON-ASSESSABLE SHARES OF THE COMMON STOCK,
                       PAR VALUE OF $0.001 PER SHARE, OF

                             NXSTAGE MEDICAL, INC.

(hereinafter, the "Corporation") transferable on the books of the Corporation
by the holder hereof in person or by duly authorized attorney upon surrender of
this certificate properly endorsed.

     This certificate and the shares represented hereby are issued and shall be
held subject to all of the provisions of the Certificate of Incorporation and
Bylaws of the Corporation and all amendments thereto, copies of which are on
file with the Transfer Agent, to all of which the holder of this certificate by
acceptance hereof assents. This certificate is not valid unless countersigned
and registered by the Transfer Agent and Registrar.

     WITNESS the facsimile seal of the Corporation and the facsimile signatures
of the duly authorized officers of the Corporation.

Dated:

/s/ Winifred L. Swan     (NXSTAGE MEDICAL, INC. SEAL)     /s/ Jeffery H. Burbank
     SECRETARY                                                  PRESIDENT

COUNTERSIGNED AND REGISTERED:
    EQUISERVE TRUST COMPANY, N.A.
        TRANSFER AGENT AND REGISTRAR
           /S/ (ILLEGIBLE)
              AUTHORIZED SIGNATURE

                                 [CERTIFICATE]
<PAGE>
<TABLE>
<S>                                                   <C>
                                          NXSTAGE MEDICAL, INC.

     THE CORPORATION HAS MORE THAN ONE CLASS OF STOCK AUTHORIZED TO BE ISSUED. THE CORPORATION WILL
FURNISH WITHOUT CHARGE TO EACH STOCKHOLDER UPON WRITTEN REQUEST A COPY OF THE FULL TEXT OF THE
PREFERENCES, VOTING POWERS, QUALIFICATIONS AND SPECIAL AND RELATIVE RIGHTS OF THE SHARES OF EACH CLASS
OF STOCK (AND ANY SERIES THEREOF) AUTHORIZED TO BE ISSUED BY THE CORPORATION AS SET FORTH IN THE
CERTIFICATE OF INCORPORATION OF THE CORPORATION AND AMENDMENTS THERETO FILED WITH THE SECRETARY OF THE
STATE OF DELAWARE.

     The following abbreviations, when used in the inscription on the face of this certificate, shall be
construed as though they were written out in full according to applicable laws or regulations:

TEN COM - as tenants in common                        UNIF GIFT MIN ACT- ____________ Custodian ________
TEN ENT - as tenants by the entireties                                      (Cust)             (Minor)
JT TEN  - as joint tenants with right                                    under Uniform Gifts to Minors
          of survivorship and not as                                     Act ___________________________
          tenants in common                                                            (State)

                 Additional abbreviations may also be used though not in the above list.

FOR VALUE RECEIVED, _________________ hereby sells, assigns and transfers unto

PLEASE INSERT SOCIAL SECURITY OR OTHER
    IDENTIFYING NUMBER OF ASSIGNEE
--------------------------------------

--------------------------------------

________________________________________________________________________________________________________
                 (PLEASE PRINT OR TYPEWRITE NAME AND ADDRESS, INCLUDING ZIP CODE, OF ASSIGNEE)

________________________________________________________________________________________________________

________________________________________________________________________________________________________

_________________________________________________________________________________________________ Shares
of the capital stock represented by this Certificate, and does hereby irrevocably constitute and
appoint

_______________________________________________________________________________________________ Attorney
to transfer the said stock on the books of the Corporation with full power of substitution in the
premises.

Dated _____________________

                                                     X
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                                                     X
                                                     ---------------------------------------------------
                                             NOTICE: THE SIGNATURE(S) TO THIS ASSIGNMENT MUST CORRESPOND
                                                     WITH THE NAME(S) AS WRITTEN UPON THE FACE OF THE
                                                     CERTIFICATE IN EVERY PARTICULAR, WITHOUT ALTERATION
                                                     OR ENLARGEMENT OR ANY CHANGE WHATEVER.

SIGNATURE(S) GUARANTEED:

By
---------------------------------------------------
THE SIGNATURE(S) MUST BE GUARANTEED BY AN
ELIGIBLE GUARANTOR INSTITUTION (BANKS,
STOCKBROKERS, SAVINGS AND LOAN ASSOCIATIONS AND
CREDIT UNIONS WITH MEMBERSHIP IN AN APPROVED
SIGNATURE GUARANTEE MEDALLION PROGRAM), PURSUANT
TO S.E.C. RULE 17Ad-15.

</TABLE><PAGE>
                                                                    Exhibit 10.1

                              NXSTAGE MEDICAL, INC.

                        1999 STOCK OPTION AND GRANT PLAN

SECTION 1.        GENERAL PURPOSE OF THE PLAN: DEFINITIONS

      The name of the plan is the NxSTAGE MEDICAL, INC. 1999 Stock Option and
Grant Plan (the "Plan"). The purpose of the Plan is to encourage and enable the
officers, employees, directors, consultants and other key persons of NxSTAGE
MEDICAL, INC., a Massachusetts corporation (the "Company"), upon whose judgment,
initiative and efforts the Company largely depends for the successful conduct of
its business to acquire a proprietary interest in the Company. It is anticipated
that providing such persons with a direct stake in the Company's welfare will
assure a closer identification of their interests with those of the Company,
thereby stimulating their efforts on the Company's behalf and strengthening
their desire to remain with the Company.

      The following terms shall be defined as set forth below:

      "Award" or "Awards," except where referring to a particular category of
grant under the Plan, shall include Incentive Stock Options, Non-Qualified Stock
Options, Restricted Stock Awards, Unrestricted Stock Awards, or any combination
of the foregoing.

      "Board" means the Board of Directors of the Company.

      "Cause" means a vote of the Board of Directors of the Company or the
successor entity, as the case may be, resolving that the grantee should be
dismissed as a result of (a) the commission of any act by a Grantee constituting
financial dishonesty against the Company (which act would be chargeable as a
crime under applicable law); (b) a Grantee's engaging in any other act of
dishonesty, fraud, intentional misrepresentation, moral turpitude, illegality or
harassment which, as determined in good faith by the board of directors of the
Company, would: (i) materially adversely affect the business or the reputation
of the Company with is current or prospective customers, suppliers, lenders
and/or other third parties with whom it does or might do business; or (ii)
negligently expose the Company to a risk of civil or criminal legal damages,
liabilities or penalties; (c) the repeated failure by a Grantee to follow the
directives of the Company's chief executive officer or board of directors or (d)
a Grantee's inadequate performance of his or her duties to the Company which
inadequate performance is no cured within 30 day after the Company has given
written notice to such Grantee specifying the inadequate performance in
reasonable detail and demanding its cure.
<PAGE>
      "Code" means the Internal Revenue Code of 1986, as amended, and any
successor Code, and related rules, regulations and interpretations.

      "Committee" has the meaning specified in Section 2.

      "Effective Date" means the date on which the Plan is approved by
stockholders as set forth at the end of this Plan.

      "Fair Market Value" of the Stock on any given date means the fair market
value of the Stock determined in good faith by the Committee; provided, however,
that (i) if the Stock is admitted to quotation on the National Association of
Securities Dealers Automated Quotation System ("NASDAQ"), the Fair Market Value
on any given date shall not be less than the average of the highest bid and
lowest asked prices of the Stock reported for such date or, if no bid and asked
prices were reported for such date, for the last day preceding such date for
which such prices were reported, or (ii) if the Stock is admitted to trading on
a national securities exchange or the NASDAQ National Market System, the Fair
Market Value on any date shall not be less than the last reported closing price
for the Stock on such exchange or system; provided further, however, that if the
date for which Fair Market Value is determined is the first day when trading
prices for the Stock are reported on NASDAQ or trading on a national securities
exchange, the Fair Market Value shall be the "Price to the Public" (or
equivalent) set forth on the cover page for the final prospectus relating to the
Company's Initial Public Offering.

      "Good Reason" means the occurrence of any of the following events: (i) a
Grantee's being subject to illegal harassment for which the Company is
responsible (ii) a relocation of more than 50 miles in a Grantee's current place
of employment; (iii) a material and comprehensive change in a Grantee's job
description and responsibilities or direct support staff size; or (iv) a
decrease of more than 15% in a Grantee's earnings from his or her earnings in
the Company's previous fiscal year, which change is not related to the Grantee's
job performance unless such decrease affects all of the Company's senior
management. For the purpose of this definition "earnings" shall not include
bonuses, unless bonuses are part of the Grantee's base pay program.

      "Incentive Stock Option" means any Stock Option designated and qualified
as an "incentive stock option" as defined in Section 422 of the Code.

      "Initial Public Offering" means the consummation of the first fully
underwritten, firm commitment public offering pursuant to an effective
registration statement under the Act, other than on Forms S-4 or S-8 or their
then equivalents, covering the offer and sale by the Company of its Stock.

      "Non-Qualified Stock Option" means any Stock Option that is not an
Incentive Stock Option.
<PAGE>
      "Option" or "Stock Option" means any option to purchase shares of Stock
granted pursuant to Section 5.

      "Restricted Stock Award" means Awards granted pursuant to Section 6.

      "Stock" means the Common Stock, par value $0.001 per share, of the
Company, subject to adjustments pursuant to Section 3.

      "Subsidiary" means any corporation or other entity (other than the
Company) in any unbroken chain of corporations or other entities beginning with
the Company if each of the corporations or entities (other than the last
corporation or entity in the unbroken chain) owns stock or other interests
possessing 50 percent or more of the economic interest or the total combined
voting power of all classes of stock or other interests in one of the other
corporations or entities in the chain.

      "Unrestricted Stock Award" means any Award granted pursuant to Section 7.

SECTION 2.  ADMINISTRATION OF PLAN: COMMITTEE AUTHORITY TO SELECT
            PARTICIPANTS AND DETERMINE AWARDS

      (a) Administration of Plan. The Plan shall be administered by the Board,
or at the discretion of the Board, by a committee or committees of the Board,
comprised, except as contemplated by Section 2(c), of not less than two
Directors. All references herein to the Committee shall be deemed to refer to
the group then responsible for administration of the Plan at the relevant time
(i.e., either the Board of Directors or a committee or committees of the Board,
as applicable).

      (b) Powers of Committee. The Committee shall have the power and authority
to grant Awards consistent with the terms of the Plan, including the power and
authority:

            (i) to select the officers, employees, directors, consultants,
advisers and key persons of the Company and its Subsidiaries to whom Awards may
from time to time be granted;

            (ii) to determine the time or times of grant, and the extent, if
any, of Incentive Stock Options, Non-Qualified Stock Options, Restricted Stock
Awards, Unrestricted Stock Awards, or any combination of the foregoing, granted
to any one or more participants;

            (iii) to determine the number of shares of Stock to be covered by
any Award;

            (iv) to determine and modify from time to time the terms and
conditions, including restrictions, not inconsistent with the terms of the Plan,
of any
<PAGE>
Award, which terms and conditions may differ among individual Awards and
participants, and to approve the form of written instruments evidencing the
Awards;

            (v) to accelerate at any time the exercisability or vesting of all
or any portion of any Award;

            (vi) to impose any limitations on Awards granted under the Plan,
including limitations on transfers, repurchase provisions and the like and to
exercise repurchase rights or obligations;

            (vii) subject to the provisions of Section 5(a)(ii), to extend at
any time the period in which Stock Options may be exercised;

            (viii) to determine at any time whether, to what extent, and under
what circumstances distribution or the receipt of Stock and other amounts
payable with respect to an Award shall be deferred either automatically or at
the election of the participant and whether and to what extent the Company shall
pay or credit amounts constituting interest (at rates determined by the
Committee) or dividends or deemed dividends on such deferrals; and

            (ix) at any time to adopt, alter and repeal such rules, guidelines
and practices for administration of the Plan and for its own acts and
proceedings as it shall deem advisable; to interpret the terms and provisions of
the Plan and any Award (including related written instruments); to make all
determinations it deems advisable for the administration of the Plan; to decide
all disputes arising in connection with the Plan; and to otherwise supervise the
administration of the Plan.

            All decisions and interpretations of the Committee shall be binding
on all persons, including the Company and Plan participants.

            (c) Delegation of Authority to Grant Awards. The Committee, in its
discretion, may delegate to the President of the Company all or part of the
Committee's authority and duties with respect to the granting of Awards at Fair
Market Value. Any such delegation by the Committee shall include a limitation as
to the amount of Awards that may be granted during the period of the delegation
and shall contain guidelines as to the determination of the exercise price of
any Option, the conversion ratio or price of other Awards and the vesting
criteria. The Committee may revoke or amend the terms of a delegation at any
time but such action shall not invalidate any prior actions of the Committee's
delegate or delegates that were consistent with the terms of the Plan.

SECTION 3.        STOCK ISSUABLE UNDER THE PLAN; MERGERS; SUBSTITUTION
<PAGE>
      (a) Stock Issuable. The maximum number of shares of Stock reserved and
available for issuance under the Plan shall be 1,200,000 shares of Common Stock,
subject to adjustment as provided in Section 3(b). For purposes of this
limitation, the shares of Stock underlying any Awards which are forfeited,
canceled, reacquired by the Company, satisfied without the issuance of Stock or
otherwise terminated (other than by exercise) shall be added back to the shares
of Stock available for issuance under the Plan. Subject to such overall
limitation, shares of Stock may be issued up to such maximum number pursuant to
any type or types of Award. The shares available for issuance under the Plan may
be authorized but unissued shares of Stock or shares of Stock reacquired by the
Company and held in its treasury.

      (b) Changes in Stock. Subject to Section 3(c) hereof, if, as a result of
any reorganization, recapitalization, reclassification, stock dividend, stock
split, reverse stock split or other similar change in the Company's capital
stock, the outstanding shares of Stock are increased or decreased or are
exchanged for a different number or kind of shares or other securities of the
Company, or additional shares or new or different shares or other securities of
the Company or other non-cash assets are distributed with respect to such shares
of Stock or other securities, the Committee shall make an appropriate or
proportionate adjustment in (i) the maximum number of shares reserved for
issuance under the Plan, (ii) the number of Stock Options that can be granted to
any one individual participant, (iii) the number and kind of shares or other
securities subject to any then outstanding Awards under the Plan, and (iv) the
exercise price and/or exchange price for each share subject to any then
outstanding Stock Options under the Plan, without changing the aggregate
exercise price (i.e., the exercise price multiplied by the number of Stock
Options) as to which such Stock Options remain exercisable. The adjustment by
the Committee shall be final, binding and conclusive. No fractional shares of
Stock shall be issued under the Plan resulting from any such adjustment, but the
Committee in its discretion may make a cash payment in lieu of fractional
shares.

      The Committee may also adjust the number of shares subject to outstanding
Awards and the exercise price and the terms of outstanding Awards to take into
consideration material changes in accounting practices or principles,
extraordinary dividends, acquisitions or dispositions of stock or property or
any other event if it is determined by the Committee that such adjustment is
appropriate to avoid distortion in the operation of the Plan, provided that no
such adjustment shall be made in the case of an Incentive Stock Option, without
the consent of the participant, if it would constitute a modification, extension
or renewal of the Option within the meaning of Section 424(h) of the Code.

      (c) Mergers and Other Sale Events. In the case of and subject to the
consummation of (i) the dissolution or liquidation of the Company, (ii) the sale
of all or substantially all of the assets of the Company on a consolidated basis
to an unrelated person or entity, (iii) a merger, reorganization or
consolidation in which the outstanding shares of stock are converted into or
exchanged for a different kind of securities of the successor entity and the
holders of the Company's outstanding voting power immediately
<PAGE>
prior to such transaction do not own a majority of the outstanding voting power
of the successor entity immediately upon completion of such transaction, or (iv)
the sale of all of the Stock of the Company to an unrelated person or entity (in
each case, regardless of the form thereof, a "Sale Event") unless otherwise
provided in the Award agreement, 50 percent (50%) of all Options that are not
vested immediately prior to the effective date of such Sale Event and 50 percent
(50%) of all Restricted Stock Awards that remain unvested immediately prior to
the effective date of such Sale Event shall vest and become fully exercisable
and/or non-forfeitable upon the effective date of such Sale Event.

      The Plan and the Awards issued hereunder shall terminate upon the
effectiveness of any such Sale Event, unless provision is made in connection
with such transaction in the sole discretion of the parties thereto for the
assumption of Awards theretofore granted (after taking into account any
acceleration hereunder), or the substitution for such Awards of new Awards of
the successor entity or a parent or subsidiary thereof, with such adjustment as
to the number and kind of shares and the per share exercise prices as such
parties shall agree (after taking into account any acceleration hereunder). In
the event of such termination, each participant shall be permitted, within a
specified period of time prior to the consummation of the Sale Event as may be
determined by the Committee, to exercise all outstanding Options held by such
participant which are then vested or will become vested upon the effectiveness
of the Sale Event, provided, however, that the exercise of Options not vested
immediately prior to the Sale Event shall be subject to the effectiveness of the
Sale Event. In the event provision is made in connection with the Sale Event for
the assumption of Awards or the substitution for such Awards of the successor
entity or a parent or subsidiary thereof, the remaining Awards that do not vest
as a result of the Sale Event shall continue to be subject to the original
vesting provisions as set forth in the Award agreements and as adjusted on a pro
rate basis to reflect the acceleration provided herein.

      Notwithstanding anything herein to the contrary in the event that
provision is made in connection with the Sale Event for the assumption or
continuation of Awards, or the substitution of such Awards, or the substitution
of such Awards with new Awards of the successor entity or parent thereof, then,
except as the committee may otherwise determine with respect to particular
Awards, any award so assumed or continued or substituted therefor shall be
deemed vested and exercisable in full upon the date on which the Grantee's
employment or service relationship with the Company and its subsidiaries or
successor entity, as the case may be, terminates if such termination occurs (i)
within twelve months after such Sale Event and (ii) such termination is by the
Company or its Subsidiaries or successor entity without Cause or by the Grantee
for Good Reason.

      (d) Substitute Awards. The Committee may grant Awards under the Plan in
substitution for stock and stock based awards held by employees, directors or
other option holders of another corporation in connection with a merger or
consolidation of the employing corporation with the Company or a Subsidiary or
the acquisition by the Company or a Subsidiary of property or stock of the
employing corporation. The Committee may direct that the substitute awards be
granted on such terms and conditions
<PAGE>
as the Committee considers appropriate in the circumstances. Any substitute
Awards granted under the Plan shall not count against the share limitation set
forth in Section 3(a).

SECTION 4.        ELIGIBILITY

      Participants in the Plan will be such full or part-time officers,
employees, directors, consultants and other key persons of the Company and its
Subsidiaries who are responsible for, or contribute to, the management, growth
or profitability of the Company and its Subsidiaries as are selected from time
to time by the Committee in its sole discretion.

SECTION 5.        STOCK OPTIONS

      Any Stock Option granted under the Plan shall be pursuant to a Stock
Option Agreement, which shall be in such form as the Committee may from time to
time approve. Option Agreements need not be identical.

      Stock Options granted under the Plan may be either Incentive Stock Options
or Non-Qualified Stock Options. Incentive Stock Options may be granted only to
employees of the Company or any Subsidiary that is a "subsidiary corporation"
within the meaning of Section 424(f) of the Code. To the extent that any Option
does not qualify as an Incentive Stock Option, it shall be deemed a
Non-Qualified Stock Option.

      No Incentive Stock Option shall be granted under the Plan after the date,
which is 10 years from the date the Plan is approved by the Board.

      (a) Terms of Stock Options. Stock Options granted under the Plan shall be
subject to the following terms and conditions and shall contain such additional
terms and conditions, not inconsistent with the terms of the Plan, as the
Committee shall deem desirable. If the Committee so determines, Stock Options
may be granted in lieu of cash compensation at the participant's election,
subject to such terms and conditions as the Committee may establish, as well as
in addition to other compensation.

            (i) Exercise Price. The exercise price per share for the Stock
covered by a Stock Option shall be determined by the Committee at the time of
grant but shall not be less than 100 percent of the Fair Market Value on the
date of grant in the case of Incentive Stock Options. If an employee owns or is
deemed to own (by reason of the attribution rules of Section 424(d) of the Code)
more than 10 percent of the combined voting power of all classes of stock of the
Company or any parent or subsidiary corporation and an Incentive Stock Option is
granted to such employee, the option price of such Incentive Stock Option shall
be not less than 110 percent of the Fair Market Value on the grant date.
<PAGE>
            (ii) Option Term. The term of each Stock Option shall be fixed by
the Committee, but no Stock Option shall be exercisable more than ten years
after the date the option is granted. If an employee owns or is deemed to own
(by reason of the attribution rules of Section 424(d) of the Code) more than 10
percent of the combined voting power of all classes of stock of the Company or
any parent or subsidiary corporation and an Incentive Stock Option is granted to
such employee, the term of such option shall be no more than five years from the
date of grant.

            (iii) Exercisability; Rights of a Stockholder. Stock Options shall
become exercisable at such time or times, whether or not in installments, as
shall be determined by the Committee at or after the grant date. The Committee
may at any time accelerate the exercisability of all or any portion of any Stock
Option. An optionee shall have the rights of a stockholder only as to shares
acquired upon the exercise of a Stock Option and not as to unexercised Stock
Options.

            (iv) Method of Exercise. Stock Options may be exercised in whole or
in part, by giving written notice of exercise to the Company, specifying the
number of shares to be purchased. Payment of the purchase price may be made by
one or more of the following methods to the extent provided in the Award
agreement: in cash, by certified or bank check, or other instrument acceptable
to the Committee in U.S. funds payable to the order of the Company in an amount
equal to the purchase price of such Option Shares.

      Payment instruments will be received subject to collection. No
certificates for Option Shares so purchased will be issued to optionee until the
Company has completed all steps required by law to be taken in connection with
the issuance and sale of the shares, including without limitation (A) receipt of
a representation from the optionee at the time of exercise of the Option that
the optionee is purchasing the Option Shares for the optionee's own account and
not with a view to any sale or distribution thereof, (B) the legending of any
certificate representing the shares to evidence the foregoing representations
and restrictions, and (C) obtaining from optionee payment or provision for all
withholding taxes due as a result of the exercise of the Option. The delivery of
certificates representing the shares of Stock to be purchased pursuant to the
exercise of a Stock Option will be contingent upon receipt from the optionee (or
a purchaser acting in his or her stead in accordance with the provisions of the
Stock Option) by the Company of the full purchase price for such shares and the
fulfillment of any other requirements contained in the Stock Option or
applicable provisions of laws.

      (b) Annual Limit on Incentive Stock Options. To the extent required for
"incentive stock option" treatment under Section 422 of the Code, the aggregate
Fair Market Value (determined as of the time of grant) of the shares of Stock
with respect to which Incentive Stock Options granted under this Plan and any
other plan of the Company or its parent and subsidiary corporations become
exercisable for the first time
<PAGE>
by an optionee during any calendar year shall not exceed $100,000. To the extent
that any Stock Option exceeds this limit, it shall constitute a Non-Qualified
Stock Option.

      (c) Non-transferability of Options. No Stock Option shall be transferable
by the optionee otherwise than by will or by the laws of descent and
distribution and all Stock Options shall be exercisable, during the optionee's
lifetime, only by the optionee, or by the optionee's legal representative or
guardian in the event of the optionee's incapacity. Notwithstanding the
foregoing, the Committee, in its sole discretion, may provide in the Award
agreement regarding a given Option that the optionee may transfer, without
consideration for the transfer, his Non-Qualified Stock Options to members of
his immediate family, to trusts for the benefit of such family members, or to
partnerships in which such family members are the only partners, provided that
the transferee agrees in writing with the Company to be bound by all of the
terms and conditions of this Plan and the applicable Option.

      (d) Termination. Unless otherwise provided in the option agreement or
determined by the Committee, upon the optionee's termination of employment (or
other business relationship) with the Company and its Subsidiaries, the
optionee's rights in his or her Stock Options shall automatically terminate.

SECTION 6.        RESTRICTED STOCK AWARDS

      (a) Nature of Restricted Stock Awards. A Restricted Stock Award is an
Award pursuant to which the Company may, in its sole discretion, grant or sell,
at par value or such other higher purchase price determined by the Committee, in
its sole discretion, shares of Stock subject to such restrictions and conditions
as the Committee may determine at the time of grant ("Restricted Stock"), which
purchase price shall be payable in cash. Conditions may be based on continuing
employment (or other business relationship) and/or achievement of
pre-established performance goals and objectives. The grant of a Restricted
Stock Award is contingent on the participant executing the Restricted Stock
Award agreement. The terms and conditions of each such agreement shall be
determined by the Committee, and such terms and conditions may differ among
individual Awards and participants.

      (b) Rights as a Stockholder. Upon execution of a written instrument
setting forth the Restricted Stock Award and payment of any applicable purchase
price, a participant shall have the rights of a stockholder with respect to the
voting of the Restricted Stock, subject to such conditions contained in the
written instrument evidencing the Restricted Stock Award. Unless the Committee
shall otherwise determine, certificates evidencing the Restricted Stock shall
remain in the possession of the Company until such Restricted Stock is vested as
provided in subsection (d) below of this Section, and the participant shall be
required, as a condition of the grant, to deliver to the Company a stock power
endorsed in blank.
<PAGE>
      (c) Restrictions. Restricted Stock may not be sold, assigned, transferred,
pledged or otherwise encumbered or disposed of except as specifically provided
herein or in the Restricted Stock Award agreement. If a participant's employment
(or other business relationship) with the Company and its Subsidiaries
terminates under the conditions specified in the relevant instrument relating to
the Award, or upon such other event or events as may be stated in the instrument
evidencing the Award, the Company or its assigns shall have the right or shall
agree, as may be specified in the relevant instrument, to repurchase some or all
of the shares of Stock subject to the Award at such purchase price as is set
forth in such instrument.

      (d) Vesting of Restricted Stock. The Committee at the time of grant shall
specify the date or dates and/or the attainment of pre-established performance
goals, objectives and other conditions on which Restricted Stock shall become
vested, subject to such further rights of the Company or its assigns as may be
specified in the instrument evidencing the Restricted Stock Award.

      (e) Waiver, Deferral and Reinvestment of Dividends. The Restricted Stock
Award agreement may require or permit the immediate payment, waiver, deferral or
investment of dividends paid on the Restricted Stock.

SECTION 7.        UNRESTRICTED STOCK AWARDS

      (a) Grant or Sale of Unrestricted Stock. The Committee may, in its sole
discretion, grant (or sell at par value or such higher purchase price determined
by the Committee) an Unrestricted Stock Award to any participant, pursuant to
which such participant may receive shares of Stock free of any vesting
restrictions ("Unrestricted Stock") under the Plan. Unrestricted Stock Awards
may be granted or sold as described in the preceding sentence in respect of past
services or other valid consideration, or in lieu of any cash compensation due
to such individual.

      (b) Elections to Receive Unrestricted Stock In Lieu of Compensation. Upon
the request of a participant and with the consent of the Committee, each such
participant may, pursuant to an advance written election delivered to the
Company no later than the date specified by the Committee, receive a portion of
the cash compensation otherwise due to such participant in the form of shares of
Unrestricted Stock either currently or on a deferred basis.

      (c) Restrictions on Transfers. The right to receive shares of Unrestricted
Stock on a deferred basis may not be sold, assigned, transferred, pledged or
otherwise encumbered, other than by will or the laws of descent and
distribution.

SECTION 8.        TAX WITHHOLDING
<PAGE>
      (a) Payment by Participant. Each participant shall, no later than the date
as of which the value of an Award or of any Stock or other amounts received
thereunder first becomes includable in the gross income of the participant for
Federal income tax purposes, pay to the Company, or make arrangements
satisfactory to the Committee regarding payment of, any federal, state, or local
taxes of any kind required by law to be withheld with respect to such income.
The Company and its Subsidiaries shall, to the extent permitted by law, have the
right to deduct any such taxes from any payment of any kind otherwise due to the
participant.

      (b) Payment in Stock. Subject to approval by the Committee, a participant
may elect to have the minimum required tax withholding obligation satisfied, in
whole or in part, by (i) authorizing the Company to withhold from shares of
Stock to be issued pursuant to any Award a number of shares with an aggregate
Fair Market Value (as of the date the withholding is effected) that would
satisfy the withholding amount due, or (ii) transferring to the Company shares
of Stock owned by the participant with an aggregate Fair Market Value (as of the
date the withholding is effected) that would satisfy the withholding amount due.

SECTION 9.        TRANSFER, LEAVE OF ABSENCE, ETC.

      For purposes of the Plan, the following events shall not be deemed a
termination of employment:

      (a) a transfer to the employment of the Company from a Subsidiary or
from the Company to a Subsidiary, or from one Subsidiary to another; or

      (b) an approved leave of absence for military service or sickness, or for
any other purpose approved by the Company, if the employee's right to
re-employment is guaranteed either by a statute or by contract or under the
policy pursuant to which the leave of absence was granted or if the Committee
otherwise so provides in writing.

SECTION 10.       AMENDMENTS AND TERMINATION

      The Board may, at any time, amend or discontinue the Plan and the
Committee may, at any time, amend or cancel any outstanding Award (or provide
substitute Awards at the same or reduced exercise or purchase price or with no
exercise or purchase price in a manner not inconsistent with the terms of the
Plan), but such price, if any, must satisfy the requirements which would apply
to the substitute or amended Award if it were then initially granted under this
Plan for the purpose of satisfying changes in law or for any other lawful
purpose, but no such action shall adversely affect rights under any outstanding
Award without the holder's consent. If and to the extent determined by the
Committee to be required by the Code to ensure that Incentive Stock Options
granted under the Plan are qualified under Section 422 of the Code, Plan
amendments shall be
<PAGE>
subject to approval by the Company's stockholders who are eligible to vote at a
meeting of stockholders. Nothing in this Section 10 shall limit the Board's or
Committee's authority to take any action permitted pursuant to Section 3(c).

SECTION 11.       STATUS OF PLAN

      With respect to the portion of any Award that has not been exercised and
any payments in cash, Stock or other consideration not received by a
participant, a participant shall have no rights greater than those of a general
creditor of the Company unless the Committee shall otherwise expressly determine
in connection with any Award or Awards. In its sole discretion, the Committee
may authorize the creation of trusts or other arrangements to meet the Company's
obligations to deliver Stock or make payments with respect to Awards hereunder,
provided that the existence of such trusts or other arrangements is consistent
with the foregoing sentence.

SECTION 12.       GENERAL PROVISIONS

      (a) No Distribution; Compliance with Legal Requirements. The Committee may
require each person acquiring Stock pursuant to an Award to represent to and
agree with the Company in writing that such person is acquiring the shares
without a view to distribution thereof. No shares of Stock shall be issued
pursuant to an Award until all applicable securities law and other legal and
stock exchange or similar requirements have been satisfied. The Committee may
require the placing of such stop-orders and restrictive legends on certificates
for Stock and Awards as it deems appropriate.

      (b) Delivery of Stock Certificates. Stock certificates to participants
under this Plan shall be deemed delivered for all purposes when the Company or a
stock transfer agent of the Company shall have mailed such certificates in the
United States mail, addressed to the participant, at the participant's last
known address on file with the Company.

      (c) Other Compensation Arrangements; No Employment Rights. Nothing
contained in this Plan shall prevent the Board from adopting other or additional
compensation arrangements, including trusts, and such arrangements may be either
generally applicable or applicable only in specific cases. The adoption of this
Plan and the grant of Awards do not confer upon any employee any right to
continued employment with the Company or any Subsidiary.

      (d) Trading Policy Restrictions. Option exercises and other Awards under
the Plan shall be subject to such Company's insider-trading-policy-related
restrictions, terms and conditions as may be established by the Committee, or in
accordance with policies set by the Committee, from time to time.
<PAGE>
SECTION 13.       EFFECTIVE DATE OF PLAN

      This Plan shall become effective upon approval by the holders of a
majority of the votes cast at a meeting of stockholders at which a quorum is
present or by written consent in accordance with applicable law. Subject to such
approval by the stockholders and to the requirement that no Stock may be issued
hereunder prior to such approval, Stock Options and other Awards may be granted
hereunder on and after adoption of this Plan by the Board.

SECTION 14.       GOVERNING LAW

      This Plan and all Awards and actions taken thereunder shall be governed by
Massachusetts law, applied without regard to conflict of law principles.

ADOPTED BY BOARD OF DIRECTORS:       September 1, 1999

APPROVED BY STOCKHOLDERS:            September 1, 1999
<PAGE>
AMENDMENT NO. 1 TO THE NxSTAGE MEDICAL, INC. 1999 STOCK OPTION AND GRANT PLAN

The NxSTAGE MEDICAL, INC. 1999 Stock Option and Grant Plan (the "Plan") is
hereby amended as follows:

      1.    Section 3(a) of the Plan is amended by increasing the number of
            shares authorized for issuance under the plan from 1.2 million to [3
            million].

      2.    All other terms and provisions of the Plan, and any Awards (as
            defined in the Plan) made thereunder, remain in full force and
            effect.

This amendment was approved by the stockholders of NxStage Medical, Inc., and
became effective, as of May 23, 2001.
<PAGE>
AMENDMENT NO. 2 TO THE NxSTAGE MEDICAL, INC. 1999 STOCK OPTION AND GRANT PLAN

The NxSTAGE MEDICAL, INC. 1999 Stock Option and Grant Plan (the "Plan") is
hereby amended as follows:

      3.    Section 3(a) of the Plan is amended by increasing the number of
            shares authorized for issuance under the plan from 3,000,000 to
            3,808,820.

      4.    All other terms and provisions of the Plan, and any Awards (as
            defined in the Plan) made thereunder, remain in full force and
            effect.

This amendment was approved by the stockholders of NxStage Medical, Inc., and
became effective, as of October 7, 2004.

<PAGE>
                  AMENDMENT NO. 3 TO THE NxSTAGE MEDICAL, INC.
                        1999 STOCK OPTION AND GRANT PLAN

The NxSTAGE MEDICAL, INC. 1999 Stock Option and Grant Plan (the "Plan") is
hereby amended as follows:

     1.   Section 3(a) of the Plan is amended by increasing the number of shares
          authorized for issuance under the plan from 3,808,820 to 5,108,820.

     2.   All other terms and provisions of the Plan, and any Awards (as defined
          in the Plan) made thereunder, remain in full force and effect.

This amendment was approved by the Board of Directors of NxStage Medical, Inc.
on September 7, 2005 and by the stockholders of NxStage Medical, Inc. on
September 14, 2005.

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