<PAGE>

EXHIBIT 4.1

                         REGISTRATION RIGHTS AGREEMENT

     This REGISTRATION RIGHTS AGREEMENT, dated as of April 7, 2000 (this
"Agreement"), is made by and among Aksys, Ltd., a Delaware corporation, with
headquarters located at Two Marriott Drive, Lincolnshire, IL 60069 (the
"Company"), and the investors named on the signature pages hereto (the "Initial
Investors").

                                   RECITALS:

     A.  In connection with the Securities Purchase Agreement dated April 7,
2000 between the Initial Investors and the Company (the "Purchase Agreement"),
the Company has agreed, upon the terms and subject to the conditions of the
Purchase Agreement, to issue and sell to the Initial Investors 1,516,697 shares
of the Company's Common Stock (the "Common Shares" or the "Securities").

     B.  In order to induce the Initial Investors to execute and deliver the
Purchase Agreement, the Company has agreed to provide certain registration
rights under the Securities Act and applicable state securities laws with
respect to the Common Shares.

     In consideration of the premises and the mutual covenants contained herein
and other good and valuable consideration, the receipt and sufficiency of which
are hereby acknowledged, the Company and the Initial Investors hereby agree as
follows:

                                   ARTICLE 1
                                  DEFINITIONS

     Capitalized terms used and not otherwise defined herein have the respective
meanings given them set forth in the Purchase Agreement.  In addition, as used
in this Agreement, the following terms have the following meanings:

     1.1  "Common Shares" means the Securities sold pursuant to the Purchase
Agreement.

     1.2  "Investors" means the Initial Investors and any of their transferees
or assignees who receive or acquire Registrable Securities and who are entitled
to the benefit of this Agreement under Article IX hereof.

     1.3  "Registrable Securities" means the Securities sold pursuant to the
Purchase Agreement and any shares of capital stock issued or issuable from time
to time (with any adjustments) in exchange for or otherwise with respect to the
Securities; provided that Securities will cease to be Registrable Securities at
such time as they have been sold under the Registration Statement or pursuant to
Rule 144.

     1.4  "Registration Period" means the period between the date of this
Agreement and the earliest of (i) the second anniversary of the date of this
Agreement, (ii) the date on which all of the Registrable Securities have been
sold by the Investors under the Registration Statement or pursuant to Rule 144
or (iii) the date on which all the Registrable Securities (in the opinion of the
<PAGE>

Investors' counsel) may be immediately sold by the Investors without
registration and without restriction as to the number of Registrable Securities
to be sold, pursuant to Rule 144 or otherwise.

     1.5  "Registration Statement" means a Registration Statement of the Company
filed under the Securities Act.

     1.6  The terms "register," "registered," and "registration" refer to a
registration effected by preparing and filing a Registration Statement or
statements in compliance with the Securities Act and pursuant to Rule 415 and
the declaration or ordering of effectiveness of such Registration Statement by
the SEC.

     1.7  "Rule 415" means Rule 415 under the Securities Act, or any successor
rule providing for offering securities on a continuous basis, and applicable
rules and regulations thereunder.

                                   ARTICLE 2
                                  REGISTRATION

     2.1  Mandatory Registration.  The Company will use its best efforts to file
with the SEC a Registration Statement on Form S-3 registering the Registrable
Securities and no other securities for resale within 30 days after the Closing
Date of the purchase of the Common Shares under the Purchase Agreement.  If Form
S-3 is not available at that time, then the Company will file a Registration
Statement on such form as is then available to effect a registration of the
Registrable Securities, subject to the consent of the Initial Investors, which
consent will not be unreasonably withheld.

     2.2  Effectiveness of the Registration Statement.  The Company will use its
best efforts to cause the Registration Statement to be declared effective by the
SEC as soon as practicable after filing, and in any event no later than the 90th
day after the Closing Date (the "Required Effective Date").  However, so long as
the Company filed the Registration Statement within 30 days after the Closing
Date, (a) if the SEC takes the position that registration of the resale of the
Registrable Securities by the Investors is not available under applicable laws,
rules and regulation and that the Company must register the offering of the
Registrable Securities as a primary offering by the Company, or (b) if the
Registration Statement receives SEC review, then the Required Effective Date
will be the 120th day after the Closing Date.  In the case of an SEC response
described in clause (a), the Company will, within 40 business days after the
date the Company receives such SEC response, file a Registration Statement as a
primary offering.  The Company's best efforts will include, but not be limited
to, promptly responding to all comments received from the staff of the SEC.  If
the Company receives notification from the SEC that the Registration Statement
will receive no action or review from the SEC, then the Company will request
that the Registration Statement become effective within five business days after
such SEC notification.  Until such time as the Registration Statement becomes
effective, the Company shall not grant any registration rights or other rights
to register securities under the Securities Act unless such rights are
subordinate to the rights of the Investors herein.
<PAGE>

     2.3  Payments by the Company.  If (i) at any time after effectiveness of
the Registration Statement, sales cannot be made thereunder during the
Registration Period for any reason, other than by reason of the operation of
Section 3.6, for a period of more than 10 consecutive business days, or 30
business days in the aggregate, during any 12-month period or (ii) the Common
Stock is not listed or included for quotation on Nasdaq, Nasdaq SmallCap, the
NYSE or AMEX for more than an aggregate of 10 business days in any 12-month
period, then the Company will thereafter make cash payments to each Investor as
partial compensation for such delay.  The amount of the cash payment made to
each Investor will be equal to 1% of the purchase price paid for the Common
Shares purchased by the Investor and not previously sold by the Investor for
each 30 business days that sales cannot be made under the effective Registration
Statement or the Common Stock is not listed or included for quotation on Nasdaq,
Nasdaq SmallCap, the NYSE or AMEX (but in no event to exceed 8% in the
aggregate).  These payments will be prorated on a daily basis during the 30
business day period and will be paid to each Investor in cash within five
business days following the end of each month as to which payment is due
hereunder.

     2.4  Effect of Late Registration.  If the Registration Statement has not
been declared effective by the Required Effective Date, then the Company will
make cash payments to each Investor as partial compensation for such delay (the
"Late Registration Payments").  The Late Registration Payments will be equal to
1% of the purchase price paid for the Common Shares purchased by such Investor
and not previously sold by such Investor for each 30 business days after the
Required Effective Date (but in no event to exceed 8% in the aggregate).  The
Late Registration Payments will be prorated on a daily basis during the 30
business day period and will be paid to the Initial Investors in cash within
five business days after the earlier of (i) the end of the 30 business days
following the Required Effective Date or (ii) the effective date of the
Registration Statement.

     2.5  Piggyback Registrations.

          (a) If, at any time prior to the expiration of the Registration
Period, a Registration Statement is not effective with respect to all of the
Registrable Securities and the Company decides to register any of its securities
for its own account or for the account of others, then the Company will promptly
give the Investors written notice thereof and will use its best efforts to
include in such registration all or any part of the Registrable Securities
requested by such Investors to be included therein (excluding any Registrable
Securities previously included in a Registration Statement); provided that
nothing in this Section 2.5 shall relieve the Company of its obligations under
Sections 2.1, 2.2 and 2.3.  This requirement does not apply to Company
registrations on Form S-4 or S-8 or their equivalents relating to equity
securities to be issued solely in connection with an acquisition of any entity
or business or equity securities issuable in connection with stock option or
other employee benefit plans or to registration statements that would otherwise
not permit the registration of resales of previously issued securities.  Each
Investor must give its request for registration under this paragraph to the
Company in writing within 15 days after receipt from the Company of notice of
such pending registration.  If the registration for which the Company gives
notice is a public offering involving an underwriting, the Company will so
advise the Investors as part of the above-described written notice.  In that
event, if the managing underwriter(s) of the public offering impose a limitation
on the number of shares of Common Stock that may be included in the Registration
Statement because, in such
<PAGE>

underwriter(s)' judgment, such limitation would be necessary to effect an
orderly public distribution, then, subject to the next sentence, the Company
will be obligated to include only such limited portion, if any, of the
Registrable Securities with respect to which such Investors have requested
inclusion hereunder. Any exclusion of Registrable Securities will be made pro
rata among all holders of Registrable Securities seeking to include shares of
Common Stock in proportion to the number of shares of Common Stock sought to be
included by those holders.

          (b) No right to registration of Registrable Securities under this
Section 2.5 limits in any way the registration required under Section 2.1 above.
The Company shall not be required to afford the Investors the opportunity to
exercise registration rights under this Section 2.5 for more than two
registrations (provided, however, that any Investor that has had any Registrable
Securities excluded from any Registration Statement in accordance with this
Section 2.5 may include in any additional Registration Statement filed by the
Company the Registrable Securities so excluded).

     2.6  Eligibility to use Form S-3.  The Company represents and warrants
that, as of the date of this Agreement, it meets the requirements for the use of
Form S-3 for registration of the resale by the Investors of the Registrable
Securities.  The Company will file all reports required to be filed by the
Company with the SEC in a timely manner so as to preserve its eligibility for
the use of Form S-3.

                                   ARTICLE 3
                     ADDITIONAL OBLIGATIONS OF THE COMPANY

     3.1  Continued Effectiveness of Registration Statement.  Subject to the
limitations set forth in Section 3.6, the Company will use its best efforts to
keep the Registration Statement covering the Registrable Securities effective
under Rule 415 at all times during the Registration Period.  In the event that
the number of shares available under a Registration Statement filed pursuant to
this Agreement is insufficient to cover all of the Registrable Securities
issued, the Company will (if permitted) amend the Registration Statement or file
a new Registration Statement (on the short form available therefor, if
applicable), or both, so as to cover all of the Registrable Securities.  The
Company will file such amendment or new Registration Statement as soon as
practicable, but in no event later than 30 business days after the necessity
therefor arises.  The Company will use its best efforts to cause such amendment
or new Registration Statement to become effective as soon as is practicable
after the filing thereof.

     3.2  Accuracy of Registration Statement.  Assuming the accuracy of
information furnished by or on behalf of the Investors, any Registration
Statement (including any amendments or supplements thereto and prospectuses
contained therein) filed by the Company covering Registrable Securities will not
contain any untrue statement of a material fact or omit to state a material fact
required to be stated therein, or necessary to make the statements therein, in
light of the circumstances in which they were made, not misleading.  The Company
will prepare and file with the SEC such amendments (including post-effective
amendments) and supplements to the Registration Statement and the prospectus
used in connection with the Registration Statement as may be necessary to permit
sales pursuant to the Registration Statement at all times during the
Registration Period (but subject to Section 3.6), and, during such period, will
comply with the provisions of the Securities Act with respect to the disposition
of all Registrable
<PAGE>

Securities of the Company covered by the Registration Statement until the
termination of the Registration Period, or if earlier, until such time as all of
such Registrable Securities have been disposed of in accordance with the
intended methods of disposition by the seller or sellers thereof as set forth in
the Registration Statement.

     3.3  Furnishing Documentation.  The Company will furnish to each Investor
whose Registrable Securities are included in a Registration Statement, or to its
legal counsel, (a) promptly after each document is filed with the SEC, one copy
of any Registration Statement filed pursuant to this Agreement and any
amendments thereto, each preliminary prospectus (if any) and final prospectus
and each amendment or supplement thereto; and (b) a number of copies of a
prospectus, including a preliminary prospectus (if any), and all amendments and
supplements thereto, and such other documents as the Investor may reasonably
request in order to facilitate the disposition of the Registrable Securities
owned by the Investor.  The Company will promptly notify by facsimile each
Investor whose Registrable Securities are included in any Registration Statement
of the effectiveness of the Registration Statement and any post-effective
amendment.

     3.4  Additional Obligations.  The Company will use its best efforts to (a)
register and qualify the Registrable Securities covered by a Registration
Statement under such other securities or blue sky laws of such jurisdictions as
each Investor who holds (or has the right to hold) Registrable Securities being
offered reasonably requests, (b) prepare and file in those jurisdictions any
amendments (including post-effective amendments) and supplements to such
registrations and qualifications as may be necessary to maintain their
effectiveness during the Registration Period, (c) take any other actions
necessary to maintain such registrations and qualifications in effect at all
times during the Registration Period, and (d) take any other actions reasonably
necessary or advisable to qualify the Registrable Securities for sale in such
jurisdictions.  Notwithstanding the foregoing, the Company is not required, in
connection such obligations, to (i) qualify to do business in any jurisdiction
where it would not otherwise be required to qualify but for this Section 3.4,
(ii) subject itself to general taxation in any such jurisdiction, (iii) file a
general consent to service of process in any such jurisdiction, (iv) provide any
undertakings that cause material expense or burden to the Company, or (v) make
any change in its charter or bylaws, which in each case the Board of Directors
of the Company determines to be contrary to the best interests of the Company
and its stockholders.

     3.5  Underwritten Offerings.  If the Investors who hold a majority in
interest of the Registrable Securities being offered in an offering pursuant to
a Registration Statement or any amendment or supplement thereto under this
Agreement select underwriters reasonably acceptable to the Company for such
offering, the Company will enter into and perform its obligations under an
underwriting agreement in usual and customary form including, without
limitation, customary indemnification and contribution obligations, with the
managing underwriter of such offering.

     3.6  Suspension of resale Rights.

          (a) The Company will notify (by telephone and also by facsimile and
reputable overnight courier) each Investor who holds Registrable Securities
being sold pursuant to a Registration Statement of the happening of any event of
which the Company has knowledge
<PAGE>

as a result of which the prospectus included in the Registration Statement as
then in effect includes an untrue statement of a material fact or omits to state
a material fact required to be stated therein or necessary to make the
statements therein, in light of the circumstances under which they were made,
not misleading. The Company will make such notification as promptly as
practicable after the Company becomes aware of the event (but in no event,
without the prior written consent of the Investor, will the Company disclose to
any Investor any of the facts or circumstances regarding the event), will
promptly prepare a supplement or amendment to the Registration Statement to
correct such untrue statement or omission, and will deliver a number of copies
of such supplement or amendment to each Investor as such Investor may reasonably
request.

          (b) Notwithstanding the obligations under Section 3.6(a), if in the
good faith judgment of the Company, following consultation with legal counsel,
it would be detrimental to the Company and its stockholders for resales of
Registrable Securities to be made pursuant to the Registration Statement due to
(i) the existence of a material development or potential material development
involving the Company which the Company would be obligated to disclose in the
Registration Statement, which disclosure would be premature or otherwise
inadvisable at such time or would have a Material Adverse Effect upon the
Company and its stockholders, or (ii) in the good faith judgment of the
Company's Board of Directors, it would adversely affect or require premature
disclosure of the filing of a Company-initiated registration of any class of its
equity securities, the Company will have the right to suspend the use of the
Registration Statement for a period of not more than ninety days, provided,
however, that the Company may so defer or suspend the use of the Registration
Statement no more than two times in any eighteen-month period, and provided,
further, that, after deferring or suspending the use of the Registration
Statement, the Company may not again defer or suspend the use of the
Registration Statement until a period of thirty days has elapsed after
resumption of the use of the Registration Statement.

          (c) Subject to the Company's rights under this Section 3, the Company
will use its best efforts to prevent the issuance of any stop order or other
suspension of effectiveness of a Registration Statement and, if such an order is
issued, will use its best efforts to obtain the withdrawal of such order at the
earliest possible time and the Company will promptly notify each Investor that
holds Registrable Securities being sold (or, in the event of an underwritten
offering, the managing underwriters) of the issuance of such order and the
resolution thereof.

          (d) Notwithstanding anything to the contrary contained herein or in
the Purchase Agreement, if the use of the Registration Statement is suspended by
the Company, the Company will promptly give notice of the suspension to all
Investors whose securities are covered by the Registration Statement, and will
promptly notify each such Investor as soon as the use of the Registration
Statement may be resumed.

     3.7  Review by the Investors.  The Company will permit a single firm of
legal counsel, designated by the Investors who hold a majority in interest of
the Registrable Securities being sold pursuant to a Registration Statement, to
review the Registration Statement and all amendments and supplements thereto (as
well as all requests for acceleration or effectiveness thereof) a reasonable
period of time prior to their filing with the SEC, and will not file any
document in a form to which such counsel reasonably objects, unless otherwise
required by law
<PAGE>

in the opinion of the Company's counsel; provided that the time periods set
forth in Section 2.2 shall be tolled to the extent that such legal counsel does
not deliver its final comments relating to such Registration Statement to the
Company within 3 business days after receipt of such Registration Statement. The
sections of any such Registration Statement including information with respect
to the Investors, the Investors' beneficial ownership of securities of the
Company or the Investors' intended method of disposition of Registrable
Securities must conform to the information provided to the Company by each of
the Investors.

     3.8  Comfort Letter; Legal Opinion.  At the request of the Investors who
hold a majority in interest of the Registrable Securities being sold in an
underwritten offering, and on the date that Registrable Securities are delivered
to an underwriter for sale in connection with the Registration Statement, the
Company will furnish to the Investors and the underwriters (i) a letter, dated
such date, from the Company's independent certified public accountants, in form
and substance as is customarily given by independent certified public
accountants to underwriters in an underwritten public offering, addressed to the
underwriters; and (ii) an opinion, dated such date, from counsel representing
the Company for purposes of the Registration Statement, in form and substance as
is customarily given in an underwritten public offering, addressed to the
underwriters and Investors.

     3.9  Due Diligence; Confidentiality.

          (a) The Company will make available upon reasonable advance notice
during normal business hours for inspection by any Investor whose Registrable
Securities are being sold pursuant to a Registration Statement, any underwriter
participating in any disposition pursuant to the Registration Statement, and any
attorney, accountant or other agent retained by any such Investor or underwriter
(collectively, the "Inspectors"), all pertinent financial and other records,
pertinent corporate documents and properties of the Company (collectively, the
"Records"), as each Inspector reasonably deems necessary to enable the Inspector
to exercise its due diligence responsibility.  The Company will cause its
officers, directors and employees to supply all information that any Inspector
may reasonably request for purposes of performing such due diligence.

          (b) Each Inspector will hold in confidence, and will not make any
disclosure (except to an Investor) of, any Records or other information that the
Company determines in good faith to be confidential, and of which determination
the Inspectors are so notified, unless (i) the disclosure of such Records is
necessary to avoid or correct a misstatement or omission in any Registration
Statement, (ii) the release of such Records is ordered pursuant to a subpoena or
other order from a court or government body of competent jurisdiction, (iii) the
information in such Records has been made generally available to the public
other than by disclosure in violation of this or any other agreement (to the
knowledge of the relevant Inspector), (iv) the Records or other information was
developed independently by an Inspector without breach of this Agreement, (v)
the information was known to the Inspector before receipt of such information
from the Company, or (vi) the information was disclosed to the Inspector by a
third party not under an obligation of confidentiality.  The Company is not
required to disclose any confidential information in the Records to any
Inspector unless and until such Inspector has entered into a confidentiality
agreement (in form and substance satisfactory to the Company) with the Company
with respect thereto, substantially in the form of this Section 3.9.  Nothing
<PAGE>

herein will be deemed to limit the Investor's ability to sell Registrable
Securities in a manner that is otherwise consistent with applicable laws and
regulations.

          (c) The Company will hold in confidence, and will not make any
disclosure of, information concerning an Investor provided to the Company under
this Agreement unless (i) disclosure of such information is necessary to comply
with federal or state securities laws, (ii) the disclosure of such information
is necessary to avoid or correct a misstatement or omission in any Registration
Statement, (iii) the release of such information is ordered pursuant to a
subpoena or other order from a court or governmental body of competent
jurisdiction, (iv) such information has been made generally available to the
public other than by disclosure in violation of this Agreement or any other
agreement, (v) the information was disclosed to the Company by a third party not
under an obligation of confidentiality or (vi) such Investor consents to the
form and content of any such disclosure.  If the Company learns that disclosure
of such information concerning an Investor is sought in or by a court or
governmental body of competent jurisdiction or through other means, the Company
will give prompt notice to such Investor prior to making such disclosure and
allow such Investor, at its expense, to undertake appropriate action to prevent
disclosure of, or to obtain a protective order for, such information.

     3.10  Listing.  The Company will use best efforts to (i) cause all of the
Registrable Securities covered by each Registration Statement to be listed on
each national securities exchange on which securities of the same class or
series issued by the Company are then listed, if any, if the listing of such
Registrable Securities is then permitted under the rules of such exchange, or
(ii) to the extent the securities of the same class or series are not then
listed on a national securities exchange, secure the designation and quotation
of all of the Registrable Securities covered by each Registration Statement on
Nasdaq.

     3.11  Transfer Agent; Registrar.  The Company will provide a transfer agent
and registrar, which may be a single entity, for the Registrable Securities not
later than the effective date of the Registration Statement.

     3.12  Share Certificates.  The Company will cooperate with the Investors
who hold Registrable Securities being sold and with the managing underwriter(s),
if any, to facilitate the timely preparation and delivery of certificates (not
bearing any restrictive legends) representing Registrable Securities to be
offered pursuant to a Registration Statement and will enable such certificates
to be in such denominations or amounts as the case may be, and registered in
such names as the Investors or the managing underwriter(s), if any, may
reasonably request, all in accordance with Article V of the Purchase Agreement.

     3.13  Plan of Distribution.  At the request of the Investors holding a
majority in interest of the Registrable Securities registered pursuant to a
Registration Statement, the Company will promptly prepare and file with the SEC
such amendments (including post-effective amendments) and supplements to the
Registration Statement, and the prospectus used in connection with the
Registration Statement, as may be necessary in order to change the plan of
distribution set forth in such Registration Statement.

     3.14  Securities Laws Compliance.  The Company will comply with all
applicable laws related to any Registration Statement relating to the sale of
Registrable Securities and to offering
<PAGE>

and sale of securities and with all applicable rules and regulations of
governmental authorities in connection therewith (including, without limitation,
the Securities Act, the Exchange Act and the rules and regulations promulgated
by the SEC).

     3.15  Further Assurances.  The Company will take all other reasonable
actions as any Investor or the underwriters, if any, may reasonably request to
expedite and facilitate disposition by such Investor of the Registrable
Securities pursuant to the Registration Statement.

     3.16  No Additional Selling Shareholders.  The Company will not, and will
not agree to, allow the holders of any securities of the Company to include any
of their securities in any Registration Statement under Section 2.1 hereof, or
any amendment or supplement thereto under Section 3.2 hereof, without the
consent of the holders of a majority in interest of the Registrable Securities.

                                   ARTICLE 4
                          OBLIGATIONS OF THE INVESTORS

     4.1  Investor Information.  As a condition to the obligations of the
Company to complete any registration pursuant to this Agreement with respect to
the Registrable Securities of each Investor, such Investor will furnish to the
Company such information regarding itself, the Registrable Securities held by it
and the intended method of disposition of the Registrable Securities held by it
as is reasonably required by the Company to effect the registration of the
Registrable Securities.  At least 10 business days prior to the first
anticipated filing date of a Registration Statement for any registration under
this Agreement, the Company will notify each Investor of the information the
Company requires from that Investor if the Investor elects to have any of its
Registrable Securities included in the Registration Statement.  If, by three
business days prior to the filing date, the Company has not received the
requested information from an Investor, then the Company may file the
Registration Statement without including Registrable Securities of that
Investor.

     4.2  Further Assurances.  Each Investor will cooperate with the Company, as
reasonably requested by the Company, in connection with the preparation and
filing of any Registration Statement hereunder, unless such Investor has
notified the Company in writing of such Investor's irrevocable election to
exclude all of such Investor's Registrable Securities from such Registration
Statement.

     4.3  Suspension of Sales.  Upon receipt of any notice from the Company
under Section 3.6, each Investor will immediately discontinue disposition of
Registrable Securities pursuant to the Registration Statement covering such
Registrable Securities until (i) it receives copies of a supplemented or amended
prospectus contemplated by Section 3.6(a) or (2) the Company advises the
Investor that a suspension of sales under Section 3.6(b) has terminated.  If so
directed by the Company, each Investor will deliver to the Company (at the
expense of the Company) or destroy (and deliver to the Company a certificate of
destruction) all copies in the Investor's possession (other than a limited
number of file copies) of the prospectus covering such Registrable Securities
that is current at the time of receipt of such notice.
<PAGE>

     4.4  Underwritten Offerings.

          (a) If Investors holding a majority in interest of the Registrable
Securities being registered (with the approval of the Initial Investors)
determine to engage the services of an underwriter, each Investor will enter
into and perform such Investor's obligations under an underwriting agreement, in
usual and customary form, including, without limitation, customary
indemnification and contribution obligations, with the managing underwriter of
such offering, and will take such other actions as are reasonably required in
order to expedite or facilitate the disposition of the Registrable Securities,
unless such Investor has notified the Company in writing of such Investor's
election to exclude all of its Registrable Securities from such Registration
Statement.

          (b) Without limiting any Investor's rights under Section 2.1 hereof,
no Investor may participate in any underwritten distribution hereunder unless
such Investor (a) agrees to sell such Investor's Registrable Securities on the
basis provided in any underwriting arrangements approved by the Investors
entitled hereunder to approve such arrangements, (b) completes and executes all
questionnaires, powers of attorney, indemnities, underwriting agreements and
other documents reasonably required under the terms of such underwriting
arrangements, and (c) agrees to pay its pro rata share of all underwriting
discounts and commissions and other fees and expenses of investment bankers and
any manager or managers of such underwriting, and legal expenses of the
underwriter, applicable with respect to its Registrable Securities, in each case
to the extent not payable by the Company under the terms of this Agreement.

                                   ARTICLE 5
                            EXPENSES OF REGISTRATION

     The Company will bear all reasonable expenses, other than underwriting
discounts and commissions, and transfer taxes, if any, incurred in connection
with registrations, filings or qualifications pursuant to Articles II and III of
this Agreement, including, without limitation, all registration, listing and
qualifications fees, printers and accounting fees, the fees and disbursements of
counsel for the Company, and the reasonable fees and disbursements of one firm
of legal counsel selected by the Initial Investors pursuant to Section 3.7
hereof (not to exceed $5,000).  The Investors shall reimburse the Company for
any out-of-pocket expenses the Company incurs in an underwritten offering
initiated by the Investors.

                                   ARTICLE 6
                                INDEMNIFICATION

     In the event that any Registrable Securities are included in a Registration
Statement under this Agreement:

     6.1  To the extent permitted by law, the Company will indemnify and hold
harmless each Investor that holds such Registrable Securities, any underwriter
(as defined in the Securities Act) for the Investors, any directors or officers
of such Investor or such underwriter and any person who controls such Investor
or such underwriter within the meaning of the Securities Act or the Exchange Act
(each, an "Indemnified Person") against any losses, claims, damages,
<PAGE>

expenses or liabilities (joint or several) (including in settlement of
litigation) (collectively, and together with actions, proceedings or inquiries
by any regulatory or self-regulatory organization, whether commenced or
threatened in respect thereof, "Claims") to which any of them become subject
under the Securities Act, the Exchange Act or otherwise, insofar as such Claims
arise out of or are based upon any of the following statements, omissions or
violations in a Registration Statement filed pursuant to this Agreement, any
post-effective amendment thereof or any prospectus included therein: (a) any
untrue statement or alleged untrue statement of a material fact contained in the
Registration Statement or any post-effective amendment thereof or the omission
or alleged omission to state therein a material fact required to be stated
therein or necessary to make the statements therein not misleading, (b) any
untrue statement or alleged untrue statement of a material fact contained in the
prospectus (as it may be amended or supplemented) or the omission or alleged
omission to state therein any material fact necessary to make the statements
made therein, in light of the circumstances under which the statements therein
were made, not misleading, or (c) any violation or alleged violation by the
Company of the Securities Act, the Exchange Act or any other law, including
without limitation any state securities law or any rule or regulation thereunder
(the matters in the foregoing clauses (a) through (c) being, collectively,
"Violations"). Subject to the restrictions set forth in Section 6.3 with respect
to the number of legal counsel, the Company will reimburse the Investors and
each such underwriter or controlling person and each such other Indemnified
Person, promptly as such expenses are incurred and are due and payable, for any
legal fees or other reasonable expenses incurred by them in connection with
investigating or defending any Claim. Notwithstanding anything to the contrary
contained herein, the indemnification agreement contained in this Section 6.1
with respect to any Indemnified Person: (i) does not apply to the extent such
Claim arises out of or is based upon a Violation that occurs in reliance upon
and in conformity with information furnished in writing to the Company by such
Indemnified Person expressly for use in connection with the preparation of the
Registration Statement or any such amendment thereof or supplement thereto, if
such prospectus was timely made available by the Company pursuant to Section 3.3
hereof; (ii) does not apply to a Claim arising out of or based on any failure by
such Indemnified Person to comply with prospectus delivery requirements (or the
Securities Act, the Exchange Act or any other law or legal requirement
applicable to them) or any covenant or agreement contained in the Purchase
Agreement or this Agreement; and (iii) does not apply to amounts paid in
settlement of any Claim if such settlement is made without the prior written
consent of the Company, which consent will not be unreasonably withheld. This
indemnity obligation will remain in full force and effect regardless of any
investigation made by or on behalf of the Indemnified Persons and will survive
the transfer of the Registrable Securities by the Investors under Article IX of
this Agreement.

     6.2  In connection with any Registration Statement in which an Investor is
participating, each such Investor will indemnify and hold harmless, severally
and not jointly, to the same extent and in the same manner set forth in Section
6.1 above, the Company, each of its directors, each of its officers who signs
the Registration Statement, each person, if any, who controls the Company within
the meaning of the Securities Act or the Exchange Act, and any other stockholder
selling securities pursuant to the Registration Statement or any of its
directors or officers or any person who controls such stockholder within the
meaning of the Securities Act or the Exchange Act (each an "Indemnified Person")
against any Claim to which any of them may become subject under the Securities
Act, the Exchange Act or otherwise, insofar as such Claim arises out of or is
based upon any of the following: (a) any matter of the type referred to
<PAGE>

clause (a) or (b) in Section 6.1 above in each case to the extent (and only to
the extent) that such violation occurs in reliance upon and in conformity with
written information furnished to the Company by such Investor expressly for use
in connection with such Registration Statement or (b) any failure by such
Investor to comply with prospectus delivery requirements (or the Securities Act,
the Exchange Act or any other law or legal requirement applicable to sales by
such Investor under the Registration Statement) or any covenant or agreement
contained in the Purchase Agreement or this Agreement with respect to sales
under the Registration Statement. Subject to the restrictions set forth in
Section 6.3, such Investor will promptly reimburse any legal or other expenses
(promptly as such expenses are incurred and due and payable) reasonably incurred
by them in connection with investigating or defending any such Claim. However,
the indemnity agreement contained in this Section 6.2 does not apply to amounts
paid in settlement of any Claim if such settlement is effected without the prior
written consent of such Investor, which consent will not be unreasonably
withheld, and no Investor will be liable under this Agreement (including this
Section 6.2 and Article VII) for the amount of any Claim that exceeds the net
proceeds actually received by such Investor as a result of the sale of
Registrable Securities pursuant to such Registration Statement. This indemnity
will remain in full force and effect regardless of any investigation made by or
on behalf of an Indemnified Party and will survive the transfer of the
Registrable Securities by the Investors under Article IX of this Agreement.

     6.3  Promptly after receipt by an Indemnified Person under this Article VI
of notice of the commencement of any action (including any governmental action),
such Indemnified Person will, if a Claim in respect thereof is to be made
against any indemnifying party under this Article VI, deliver to the
indemnifying party a written notice of the commencement thereof.  The
indemnifying party may participate in, and, to the extent the indemnifying party
so desires, jointly with any other indemnifying party similarly given notice,
assume control of the defense thereof with counsel mutually satisfactory to the
indemnifying parties and the Indemnified Person.  In that case, the indemnifying
party will diligently pursue such defense.  If, in the reasonable opinion of
counsel retained by the indemnifying party, the representation by such counsel
of the Indemnified Person and the indemnifying party would be inappropriate due
to actual or potential conflicts of interest between the Indemnified Person and
any other party represented by such counsel in such proceeding or the actual or
potential defendants in, or targets of, any such action including the
Indemnified Person and such Indemnified Person reasonably determines that there
may be legal defenses available to such Indemnified Person that are different
from or in addition to those available to the indemnifying party, then the
Indemnified Person is entitled to assume such defense and may retain its own
counsel, with the fees and expenses to be paid by the indemnifying party
(subject to the restrictions on settlement under Section 6.1 or 6.2, as
applicable).  The Company will pay for only one separate legal counsel for the
Investors collectively, and such legal counsel will be selected by the Investors
holding a majority in interest of the Registrable Securities.  The failure to
deliver written notice to the indemnifying party within a reasonable time of the
commencement of any such action does not relieve an indemnifying party of any
liability to an Indemnified Person under this Article VI, except to the extent
that the indemnifying party is prejudiced in its ability to defend such action.
The indemnification required by this Article VI will be made by periodic
payments of the amount thereof during the course of the investigation or
defense, as such expense, loss, damage or liability is incurred and is due and
payable.
<PAGE>

                                   ARTICLE 7
                                 CONTRIBUTION

     To the extent that any indemnification provided for herein is prohibited or
limited by law, the indemnifying party will make the maximum contribution with
respect to any amounts for which it would otherwise be liable under Article VI
to the fullest extent permitted by law.  However, (a) no contribution will be
made under circumstances where the maker would not have been liable for
indemnification under the fault standards set forth in Article VI, (b) no seller
of Registrable Securities guilty of fraudulent misrepresentation (within the
meaning of Section 11(f) of the Securities Act) will be entitled to contribution
from any seller of Registrable Securities who was not guilty of such fraudulent
misrepresentation, and (c) contribution (together with any indemnification or
other obligations under this Agreement) by any seller of Registrable Securities
will be limited in amount to the net amount of proceeds received by such seller
from the sale of such Registrable Securities.

                                   ARTICLE 8
                             EXCHANGE ACT REPORTING

     In order to make available to the Investors the benefits of Rule 144 or any
similar rule or regulation of the SEC that may at any time permit the Investors
to sell securities of the Company to the public without registration, the
Company will:

          (a) File with the SEC in a timely manner, and make and keep available,
all reports and other documents required of the Company under the Securities Act
and the Exchange Act so long as the Company remains subject to such requirements
(it being understood that nothing herein limits the Company's obligations under
Section 4.3 of the Purchase Agreement) and the filing and availability of such
reports and other documents is required for the applicable provisions of Rule
144; and

          (b) Furnish to each Investor, so long as such Investor holds
Registrable Securities, promptly upon the Investor's request, (i) a written
statement by the Company that it has complied with the reporting requirements of
the Securities Act and the Exchange Act, (ii) a copy of the most recent annual
or quarterly report of the Company and such other reports and documents filed by
the Company with the SEC and (iii) such other information as may be reasonably
requested to permit the Investors to sell such securities pursuant to Rule 144
without registration.

                                   ARTICLE 9
                       ASSIGNMENT OF REGISTRATION RIGHTS

     The rights of the Investors hereunder, including the right to have the
Company register Registrable Securities pursuant to this Agreement, will be
automatically assigned by the Investors to transferees or assignees of all or
any portion of the Registrable Securities, but only if (a) the Investor agrees
in writing with the transferee or assignee to assign such rights, and a copy of
such agreement is furnished to the Company within a reasonable time after such
assignment, (b) the Company is, within a reasonable time after such transfer or
assignment, furnished with written notice of the name and address of such
transferee or assignee and the securities with
<PAGE>

respect to which such registration rights are being transferred or assigned, (c)
such transfer or assignment was not made under the Registration Statement or
Rule 144, (d) at or before the time the Company received the written notice
contemplated by clause (b) of this sentence, the transferee or assignee agrees
in writing with the Company to be bound by all of the provisions contained
herein, (e) such transfer is made in accordance with the applicable requirements
of the Purchase Agreement and (f) the transferee is an "accredited investor" as
that term is defined in Rule 501 of Regulation D. Any transferee or assignee of
an Investor under Article IX shall be deemed an "Investor" for all purposes of
this Agreement, and shall be entitled to all rights of, and subject to all
obligations (including indemnification obligations) of, an Investor hereunder.

                                   ARTICLE 10
                        AMENDMENT OF REGISTRATION RIGHTS

     This Agreement may be amended and the obligations hereunder may be waived
(either generally or in a particular instance, and either retroactively or
prospectively) only with the written consent of the Company and of the Investors
who then hold a majority in interest of the Registrable Securities (but not
including any Investor who is not affected by such amendment or waiver).  Any
amendment or waiver effected in accordance with this Article X is binding upon
each Investor and the Company.  Notwithstanding the foregoing, no amendment or
waiver will retroactively affect any Investor without its consent, or will
prospectively adversely affect any Investor who no longer owns any Registrable
Securities without its consent.  Neither Article VI nor Article VII hereof may
be amended or waived in a manner adverse to an Investor without its consent.

                                   ARTICLE 11
                                 MISCELLANEOUS

     11.1  Conflicting Instructions. A person or entity is deemed to be a holder
of Registrable Securities whenever such person or entity owns of record such
Registrable Securities.  If the Company receives conflicting instructions,
notices or elections from two or more persons or entities with respect to the
same Registrable Securities, the Company will act upon the basis of
instructions, notice or election received from the registered owner of such
Registrable Securities.

     11.2  Notices.  Any notices required or permitted to be given under the
terms of this Agreement will be given as set forth in the Purchase Agreement.

     11.3  Waiver.  Failure of any party to exercise any right or remedy under
this Agreement or otherwise, or delay by a party in exercising such right or
remedy, does not operate as a waiver thereof.

     11.4  Governing Law.  This Agreement will be governed by and interpreted in
accordance with the laws of the State of Delaware without regard to the
principles of conflict of laws.  The parties hereto hereby submit to the
exclusive jurisdiction of the United States federal and state courts located in
the State of Delaware with respect to any dispute arising under this Agreement,
the agreements entered into in connection herewith or the transactions
contemplated hereby or thereby.
<PAGE>

     11.5  Severability.  If any provision of this Agreement is invalid or
unenforceable under any applicable statute or rule of law, then such provision
will be deemed modified in order to conform with such statute or rule of law.
Any provision hereof that may prove invalid or unenforceable under any law will
not affect the validity or enforceability of any other provision hereof.

     11.6  Entire Agreement.  This Agreement and the Purchase Agreement
(including all schedules and exhibits thereto) constitute the entire agreement
among the parties hereto with respect to the subject matter hereof and thereof.
There are no restrictions, promises, warranties or undertakings, other than
those set forth or referred to herein or therein.  This Agreement supersedes all
prior agreements and understandings among the parties hereto with respect to the
subject matter hereof.

     11.7  Successors and Assigns.  Subject to the requirements of Article IX
hereof, this Agreement inures to the benefit of and is binding upon the
successors and assigns of each of the parties hereto.  Notwithstanding anything
to the contrary herein, including, without limitation, Article IX, the rights of
an Investor hereunder are assignable to and exercisable by a bona fide pledgee
of the Registrable Securities in connection with an Investor's margin or
brokerage accounts.

     11.8  Use of Pronouns.  All pronouns refer to the masculine, feminine or
neuter, singular or plural, as the context may require.

     11.9  Headings.  The headings of this Agreement are for convenience of
reference only, are not part of this Agreement and do not affect its
interpretation.

     11.10  Counterparts.  This Agreement may be executed in two or more
counterparts, each of which is deemed an original but all of which constitute
one and the same agreement.  This Agreement, once executed by a party, may be
delivered to the other party hereto by facsimile transmission, and facsimile
signatures are binding on the parties hereto.

     11.11  Further Assurances.  Each party will do and perform, or cause to be
done and performed, all such further acts and things, and will execute and
deliver all other agreements, certificates, instruments and documents, as
another party may reasonably request in order to carry out the intent and
accomplish the purposes of this Agreement and the consummation of the
transactions contemplated hereby.

     11.12  Consents.  All consents and other determinations to be made by the
Investors pursuant to this Agreement will be made by the Initial Investors or
the Investors holding a majority in interest of the Registrable Securities.

     11.13  No Strict Construction.  The language used in this Agreement is
deemed to be the language chosen by the parties to express their mutual intent,
and no rules of strict construction will be applied against any party.
<PAGE>

     IN WITNESS WHEREOF, the undersigned Investors and the Company have caused
this Agreement to be duly executed as of the date first above written.

                              COMPANY:
                              AKSYS, LTD.



                              By:  /s/ William C. Dow
                                   ------------------
                                   Title: President and CEO

                              INVESTORS:

                              /s/ Safeco Resource Series Trust
                              --------------------------------

                              /s/ Safeco Common Stock Trust
                              -----------------------------

                              /s/ Narragansett I, LP
                              ----------------------

                              /s/ Narragansett Offshore, Ltd.
                              -------------------------------

                              /s/ Western United Life Assurance Co.
                              -------------------------------------

                              /s/ Special Situations Cayman Fund, LP
                              --------------------------------------

                              /s/ Special Situations Private Equity Fund, LP
                              ----------------------------------------------

                              /s/ Special Situations Fund III, LP
                              -----------------------------------