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                                                                    Exhibit 4.21


                         REGISTRATION RIGHTS AGREEMENT

    This REGISTRATION RIGHTS AGREEMENT (the "Agreement"), is made as of the
March 16, 2000, by and among Rhythms NetConnections Inc., a Delaware corporation
(the "Company"), and the entities listed on Schedule I to this Agreement.

    WHEREAS, the Company and HMTF Bridge RHY, LLC entered into a Preferred
Stock and Warrant Purchase Agreement dated as of February 6, 2000 (the "Stock
Purchase Agreement");

    WHEREAS, pursuant to an Assignment and Assumption, the Holders (as defined
below) have become parties to the Stock Purchase Agreement;

    WHEREAS, it is a condition precedent to the closing of the transactions
contemplated in the Stock Purchase Agreement that the parties hereto execute and
deliver this Agreement;

    NOW THEREFORE, in consideration of the premises, mutual promises and
covenants contained in this Agreement and intending to be legally bound, the
parties hereto hereby agree as follows:

                                   ARTICLE I

                                  Definitions
                                  -----------
    SECTION 1.01. Definitions. Terms defined in the Stock Purchase Agreement are
used herein as therein defined. In addition, the following terms, as used
herein, have the following meanings:

    "Commission" means the Securities and Exchange Commission.

    "Demand Registration" means a registration under the Securities Act
requested in accordance with Section 2.01.

    "Holders" means the entities set forth on Schedule I (including any
affiliates thereof) and any direct or indirect transferee of any Registrable
Securities held by such entities.

    "Investor Rights Agreement" means the Amended and Restated Investors'
Rights Agreement dated as of April 6, 1999 among the Company and certain
investors parties thereto.

    "Investor Rights Agreement 30% Right" means the rights to participate in a
Company registration pursuant to Section 1.8 of the Investor Rights Agreement,
pursuant to which the amount of securities of the selling holders under such
agreement to be included in a piggyback registration shall not, except in
certain circumstances, be reduced below thirty percent (30%) of the total amount
of securities included in such offering.

    "Piggyback Registration" has the meaning set forth in Section 2.02.

    "Registrable Common Stock" means the shares of Common Stock issued upon
conversion of the Registrable Series E Preferred Stock or upon exercise of the
Warrants, plus any

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additional shares of Common Stock issued in respect thereof in connection with
any stock split, stock dividend or similar event with respect to the Common
Stock.

    "Registrable Series E Preferred Stock" means the Series E Preferred Stock
purchased pursuant to the Stock Purchase Agreement, plus any additional shares
of Series E Preferred Stock issued in respect thereof in connection with any
stock split, stock dividend or similar event with respect to the Series E
Preferred Stock.

    "Registrable Securities" means (a) the Registrable Series E Preferred Stock,
(b) the Registrable Common Stock and (c) any securities of the Company or any
successor entity into which Registrable Common Stock or Registrable Series E
Preferred Stock may hereafter be converted or changed. As to any particular
Registrable Securities, such securities shall cease to be Registrable Securities
when (i) a registration statement with respect to the sale of such securities
shall have become effective under the Securities Act and such securities shall
have been disposed of under such registration statement, (ii) such securities
shall have been transferred pursuant to Rule 144, (iii) such securities shall
have been otherwise transferred or disposed of, and new certificates therefor
not bearing a legend restricting further transfer shall have been delivered by
the Company, and subsequent transfer or disposition of them shall not require
their registration or qualification under the Securities Act or any similar
state law then in force or (iv) such securities shall have ceased to be
outstanding.

    "Requesting Holders" means the Holders requesting a Demand Registration,
and shall include parties deemed "Requesting Holders" pursuant to Section
2.01(a)(iii).

    "Rule 144" means Rule 144 (or any successor rule of similar effect)
promulgated under the Securities Act.

    "Selling Holder" means any Holder who is selling Registrable Securities
pursuant to a public offering registered hereunder.

    "Senior Piggyback Registration Rights" means those piggyback registration
rights pursuant to (i) the Warrant Registration Rights Agreement dated as of May
5, 1998 among the Company and parties thereto, (ii) the Warrant to Purchase
Shares of Common Stock granted to Sun Financial Group dated as of May 19, 1998,
(iii) the Warrant to Purchase Shares of Common Stock granted to GATX dated as of
March 31, 1999 and (iv) the Stock Subscription Warrant granted to Cisco Systems
Capital Corporation dated as of April 5, 1999.

    "Shelf Registration" has the meaning set forth in Section 2.03(b).

    "Underwriter" means a securities dealer who purchases any Registrable
Securities as principal and not as part of such dealer's market-making
activities.

    SECTION 1.02. Internal References. Unless the context indicates otherwise,
references to Articles, Sections and paragraphs shall refer to the corresponding
articles, sections and paragraphs in this Agreement, and references to the
parties shall mean the parties to the Stock Purchase Agreement.

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                                  ARTICLE II

                              Registration Rights
                              -------------------

    SECTION 2.01. Demand Registration. (a) (i) Holders of a majority of the
Registrable Securities held by the Holders may make up to four (4) written
requests for a Demand Registration under this Section 2.01 of all or any part of
the Registrable Securities held by such Holders; provided, that (A) each such
Demand Registration by the Holders must be in respect of Registrable Securities
with a fair market value of at least $50,000,000, (B) the Holders shall not be
entitled to a Demand Registration if, during the 120 days preceding such
request, the Holders had requested a Demand Registration and (C) unless and
until the consent described in Section 5.10 is obtained, the Holders shall not
be entitled to a Demand Registration which results in such registration
statement being declared effective within 120 days after the effective date of
any registration effected pursuant to Section 1.2 of the Investors Rights
Agreement.

         (ii)  Any request for a Demand Registration will specify the aggregate
number of shares of Registrable Securities proposed to be sold by the Requesting
Holders and will also specify the intended method of disposition thereof. A
registration will not count as a Demand Registration until it has become
effective. Should a Demand Registration not become effective due to the failure
of a Holder to perform its obligations under this Agreement or the inability of
the Requesting Holders to reach agreement with the Underwriters for the proposed
sale on price or other customary terms for such transaction, or in the event the
Requesting Holders withdraw or do not pursue the request for the Demand
Registration (in each of the foregoing cases, provided that at such time the
Company is in compliance in all material respects with its obligations under
this Agreement), then, subject to Section 2.01(b), such Demand Registration
shall be deemed to have been effected (provided that (i) if, the Demand
Registration does not become effective because a material adverse change has
occurred, or is reasonably likely to occur, in the condition (financial or
otherwise), business, assets or results of operations of the Company and its
subsidiaries taken as a whole subsequent to the date of the written request made
by the Requesting Holders, (ii) if the Company withdraws the Demand Registration
for any reason or (iii) if, after the Demand Registration has become effective,
an offering of Registrable Securities pursuant to a registration is interfered
with by any stop order, injunction, or other order or requirement of the
Commission or other governmental agency or court then the Demand Registration
shall not be deemed to have been effected and will not count as a Demand
Registration).

         (iii) Upon receipt of any request for a Demand Registration by holders
of a majority of the Registrable Securities, the Company shall promptly (but in
any event within ten (10) days) give written notice of such proposed Demand
Registration to all other Holders, and all such Holders shall have the right,
exercisable by written notice to the Company within twenty (20) days of their
receipt of the Company's notice, to elect to include in such Demand Registration
such portion of their Registrable Securities as they may request. All such
Holders requesting to have their Registrable Securities included in a Demand
Registration in accordance with the preceding sentence shall be deemed to be
"Requesting Holders" for purposes of this Section 2.01.

    (b)  In the event that the Requesting Holders withdraw or do not pursue a
request for a Demand Registration and, pursuant to Section 2.01(a) hereof, such
Demand Registration is

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deemed to have been effected, the Holders may reacquire such Demand Registration
(such that the withdrawal or failure to pursue a request will not count as a
Demand Registration hereunder) if the Selling Holders reimburse the Company for
any and all Registration Expenses incurred by the Company in connection with
such request for a Demand Registration.

    (c)  If the Requesting Holders so elect, the offering of such Registrable
Securities pursuant to such Demand Registration shall be in the form of a "firm
commitment" underwritten offering. A majority in interest of the Requesting
Holders shall have the right to select the managing Underwriters and any
additional investment bankers and managers to be used in connection with any
offering under this Section 2.01, subject to the Company's approval, which
approval shall not be unreasonably withheld.

    (d)  The Requesting Holders will inform the Company of the time and manner
of any disposition of Registrable Common Stock, and agree to reasonably
cooperate with the Company in effecting the disposition of the Registrable
Common Stock in a manner that does not unreasonably disrupt the public trading
market for the Common Stock; provided, however, that the Holders' only right to
a shelf registration statement shall be pursuant to Section 2.03.

    (e)  Subject to the Investor Rights Agreement 30% Right, no securities to be
sold for the account of any Person (including the Company) other than a
Requesting Holder shall be included in a Demand Registration unless the managing
Underwriter or Underwriters shall advise the Company and the Requesting Holders
in writing that the inclusion of such securities will not materially and
adversely affect the price of the offering (a "Material Adverse Effect").
Furthermore, in the event the managing Underwriter or Underwriters shall advise
the Company or the Requesting Holders that even after exclusion of all
securities of other Persons (including the Company) pursuant to the immediately
preceding sentence, the amount of Registrable Securities proposed to be included
in such Demand Registration by Requesting Holders is sufficiently large to cause
a Material Adverse Effect, the Registrable Securities of the Requesting Holders
to be included in such Demand Registration shall equal the number of shares
which the Company and the Requesting Holders are so advised can be sold in such
offering without a Material Adverse Effect and such shares shall be allocated
pro rata among the Requesting Holders on the basis of the number of Registrable
Securities requested to be included in such registration by each such Requesting
Holder; provided, however, that if any Registrable Securities requested to be
registered pursuant to a Demand Registration under Section 2.01 are excluded
from registration hereunder, then the Holder(s) having shares excluded
("Excluded Holders") shall have the right to withdraw all, or any part, of their
shares from such registration.

    SECTION 2.02. Piggyback Registration. (a) If the Company proposes to file a
registration statement under the Securities Act with respect to an offering of
Common Stock for its own account or for the account of another Person (other
than a registration statement on Form S-4 or S-8, or, except as provided for in
Section 2.03, pursuant to Rule 415 (or any substitute form or rule,
respectively, that may be adopted by the Commission)), the Company shall give
written notice of such proposed filing to the Holders at the address set forth
in the share register of the Company as soon as reasonably practicable (but in
no event less than 15 days before the anticipated filing date), undertaking to
provide each Holder the opportunity to register on the same terms and conditions
such number of shares of Registrable Common Stock as such Holder may request (a
"Piggyback Registration"). Each Holder will have seven business days after
receipt of any such notice to notify the Company as to whether it wishes to
participate in a

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Piggyback Registration (which notice shall not be deemed to be a request for a
Demand Registration); provided that should a Holder fail to provide timely
notice to the Company, such Holder will forfeit any rights to participate in the
Piggyback Registration with respect to such proposed offering. In the event that
the registration statement is filed on behalf of a Person other than the
Company, the Company will use its best efforts to have the shares of Registrable
Common Stock that the Holders wish to sell included in the registration
statement. If the Company or the Person for whose account such offering is being
made shall determine in its sole discretion not to register or to delay the
proposed offering, the Company may, at its election, provide written notice of
such determination to the Holders and (i) in the case of a determination not to
effect the proposed offering, shall thereupon be relieved of the obligation to
register such Registrable Common Stock in connection therewith, and (ii) in the
case of a determination to delay a proposed offering, shall thereupon be
permitted to delay registering such Registrable Common Stock for the same period
as the delay in respect of the proposed offering. As between the Company and the
Selling Holders, the Company shall be entitled to select the Underwriters in
connection with any Piggyback Registration.

    (b)  If the Registrable Securities requested to be included in the Piggyback
Registration by any Holder differ from the type of securities proposed to be
registered by the Company and the managing Underwriter advises the Company that
due to such differences the inclusion of such Registrable Securities would cause
a Material Adverse Effect, then (i) the number of such Holders' Registrable
Securities to be included in the Piggyback Registration shall be reduced to an
amount which, in the opinion of the managing Underwriter, would eliminate such
Material Adverse Effect or (ii) if no such reduction would, in the opinion of
the managing Underwriter, eliminate such Material Adverse Effect, then the
Company shall have the right to exclude all such Registrable Securities from
such Piggyback Registration, provided, that no other securities of such type are
included and offered for the account of any other Person in such Piggyback
Registration. Any partial reduction in number of Registrable Securities of any
Holder to be included in the Piggyback Registration pursuant to clause (i) of
the immediately preceding sentence shall be effected pro rata based on the ratio
which such Holder's requested shares bears to the total number of shares
requested to be included in such Piggyback Registration by all Persons other
than the Company who have the contractual right to request that their shares be
included in such registration statement and who have requested that their shares
be included, subject to the Senior Piggyback Registration Rights and (if
applicable) the Investor Rights Agreement 30% Right. If the Registrable
Securities requested to be included in the registration statement are of the
same type as the securities being registered by the Company and the managing
Underwriter advises the Company that the inclusion of such Registrable
Securities would cause a Material Adverse Effect, the Company will be obligated
to include in such registration statement, as to each Holder only a portion of
the shares such Holder has requested be registered equal to the ratio which such
Holder's requested shares bears to the total number of shares requested to be
included in such registration statement by all Persons (other than the Person or
Persons initiating such registration request) who have the contractual right to
request that their shares be included in such registration statement and who
have requested their shares be included, subject to the Senior Piggyback
Registration Rights and the (if applicable) Investor Rights Agreement 30% Right.
If the Company initiated the registration, then the Company may include all of
its securities in such registration statement before any such Holder's requested
shares are included. If another security holder initiated the registration, then
the Company may not include any of its securities in such registration statement
unless all Registrable Securities requested to be included in the registration
statement by all Holders are included in such


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registration statement. If as a result of the provisions of this Section 2.02(b)
any Holder shall not be entitled to include all Registrable Securities in a
registration that such Holder has requested to be so included, such Holder may
withdraw such Holder's request to include Registrable Securities in such
registration statement prior to its effectiveness.

    SECTION 2.03. Shelf Registration. (a) Holders of a majority of the
Registrable Securities may, at any time after the first anniversary of the
Closing Date (the "First Anniversary"), make a written request that the Company
effect a shelf registration of a portion of the Registrable Securities held by
such Holders (the "Shelf Registration") pursuant to Rule 415; provided, that a
Holder will be entitled to include in the Shelf Registration no more than 25% of
the Registrable Securities held by such Holder immediately after the Closing
Date (giving effect to accretion of dividends as of such anniversary on such
holder's Registrable Series E Preferred Stock and exercise, as of or prior to
such anniversary, of such holder's Warrants). Upon receipt of a request for the
Shelf Registration, the Company shall promptly (but in any event within 10 days)
give written notice of the proposed Shelf Registration to all other Holders, and
each such other Holder shall have the right to include in the Shelf Registration
up to 25% of the Registrable Securities held by such Holder immediately after
the Closing Date (giving effect to accretion of dividends as of such anniversary
on such holder's Registrable Series E Preferred Stock and exercise, as of or
prior to such anniversary, of such holder's Warrants).

    (b)  From and after the third anniversary of the Closing Date, Holders of a
majority of the Registrable Securities may make a written request that the
Company amend the Shelf Registration to include in the Shelf Registration no
more than 50% of the Registrable Securities held by such Holders immediately
after the Closing Date (giving effect to accretion of dividends as of such
anniversary on such holder's Registrable Series E Preferred Stock and exercise,
as of or prior to such anniversary, of such holder's Warrants). Upon receipt of
such request, the Company shall promptly (but in any event within 10 days) give
written notice of the proposed amendment to all other Holders, and each such
other Holder shall have the right to include in the amended Shelf Registration
up to 50% of the Registrable Securities held by such Holder immediately after
the Closing Date (giving effect to accretion of dividends as of such anniversary
on such holder's Registrable Series E Preferred Stock and exercise, as of or
prior to such anniversary, of such holder's Warrants). From and after the fourth
anniversary of the Closing Date, Holders of a majority of the Registrable
Securities may make a written request that the Company amend the Shelf
Registration to include in the Shelf Registration no more than 75% of the
Registrable Securities held by such Holder immediately after the Closing Date
(giving effect to accretion of dividends as of such anniversary on such holder's
Registrable Series E Preferred Stock and exercise, as of or prior to such
anniversary, of such holder's Warrants). Upon receipt of such request, the
Company shall promptly (but in any event within 10 days) give written notice of
the proposed amendment to all other Holders, and each such other Holder shall
have the right to include in the amended Shelf Registration up to 75% of the
Registrable Securities held by such Holders immediately after the Closing Date
(giving effect to accretion of dividends as of such anniversary on such Holder's
Registrable Series E Preferred Stock and exercise, as of or prior to such
anniversary, of such Holder's Warrants). From and after the fifth anniversary of
the Closing Date, Holders of a majority of the Registrable Securities may make a
written request that the Company amend the Shelf Registration to include in the
Shelf Registration up to 100% of the Registrable Securities held by such Holders
immediately after the Closing Date (giving effect to accretion of dividends as
of such anniversary on such holder's Registrable Series E Preferred Stock and
exercise, as of or prior to such anniversary, of


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such holder's Warrants). Upon receipt of such request, the Company shall
promptly (but in any event within 10 days) give written notice of the proposed
amendment to all other Holders, and each such other Holder shall have the right
to include in the amended Shelf Registration up to 100% of the Registrable
Securities held by such Holder immediately after the Closing Date (giving effect
to accretion of dividends as of such anniversary on such holder's Registrable
Series E Preferred Stock and exercise, as of or prior to such anniversary, of
such holder's Warrants). If a Nonconsent Event (as defined in Section 5.10)
shall have occurred, references in this paragraph to the "third anniversary of
the Closing Date" shall mean the second anniversary of the Closing Date,
references to the "fourth anniversary of the Closing Date" shall mean the third
anniversary of the Closing Date and references to the "fifth anniversary of the
Closing Date" shall mean the fourth anniversary of the Closing Date.

    (c)  If the Company's ability to amend the registration statement for the
Shelf Registration to increase the number of Registrable Securities included
therein (or to file a new shelf registration statement in respect thereof) in
accordance with Section 2.03 (b) is subject to any contractual limitations that
could delay the Company's ability to file or cause to become effective such
registration statement, then, if requested by Holders of a majority of the
Registrable Securities, the Company shall, in lieu of following the procedure
set forth in Section 2.03(b), file a single registration statement for the Shelf
Registration (and cause such registration statement to become and remain
effective for the period set forth in Section 3.01) that would permit the
offering of such portion of the Registrable Securities (up to 100%) as may be
requested by the Holders of a majority of the Registrable Securities (it being
understood and agreed that the Holders of the Registrable Securities would not
have the right to offer and sell from such Shelf Registration Registrable
Securities other than in accordance with the schedule and amounts set forth in
Section 2.03(b) and that the Company would have the right to take whatever
measures necessary to ensure that such schedule was followed, including, without
limitation the issuance of stop orders).

                                  ARTICLE III

                            Registration Procedures
                            -----------------------

    SECTION 3.01. Filings; Information. In connection with the registration of
Registrable Securities pursuant to Section 2.01, Section 2.02 and Section 2.03
hereof, the Company will use its reasonable best efforts to effect the
registration of such Registrable Securities as promptly as is reasonably
practicable, and in connection with any such request:

    (a)  The Company will expeditiously prepare and file with the Commission a
registration statement on any form for which the Company then qualifies and
which counsel for the Company shall deem appropriate and available for the sale
of the Registrable Securities to be registered thereunder in accordance with the
intended method of distribution thereof, and use its reasonable best efforts to
cause such filed registration statement to become and remain effective (i) with
respect to any Demand Registration or Piggyback Registration, for such period,
not to exceed 60 days, as may be reasonably necessary to effect the sale of such
securities, (ii) with respect to the Shelf Registration, until the earlier of
the sale of all Registrable Securities thereunder and the eighth anniversary of
the Closing Date (it being understood that if at any time


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all the Registrable Securities then permitted to be sold under such Shelf
Registration pursuant to Section 2.03 have been sold but the Holders have the
right to request the addition of additional Registrable Securities to the Shelf
Registration in the future pursuant to Section 2.03, the Company may (at its
option) either cause the registration statement to remain effective
(notwithstanding the fact that all securities then registrable on such shelf
registration statement shall have been sold) and to file post-effective
amendments when required to permit the sale of the additional Registrable
Securities or prepare and file, and cause to become and remain effective, a new
shelf registration statement to effect the registration of the additional
Registrable Securities when required pursuant to Section 2.03); provided that if
the Company shall furnish to the Selling Holder a certificate signed by the
Company's Chairman, President or any Vice-President stating that the Company's
Board of Directors has determined in good faith that it would be detrimental or
otherwise disadvantageous to the Company or its shareholders for such a
registration statement to be filed as expeditiously as possible because the sale
of Registrable Securities covered by such Registration Statement or the
disclosure of information in any related prospectus or prospectus supplement
would materially interfere with any acquisition, financing or other material
event or transaction which is then intended or the public disclosure of which at
the time would be materially prejudicial to the Company, the Company may
postpone the filing or effectiveness of a registration statement for a period of
not more than 120 days; provided that during any 360-day period the Company
shall use its reasonable best efforts to permit a period of at least 120
consecutive days during which the Company will make a registration statement
available under this Agreement; and provided further that if (i) the effective
date of any registration statement filed pursuant to a Demand Registration would
otherwise be at least 45 calendar days, but fewer than 90 calendar days, after
the end of the Company's fiscal year, and (ii) the Securities Act requires the
Company to include audited financials as of the end of such fiscal year, the
Company may delay the effectiveness of such registration statement for such
period as is reasonably necessary to include therein its audited financial
statements for such fiscal year. If the Company exercises its right to postpone
the filing or effectiveness of a registration statement, the applicable
Requesting Holders shall be entitled to withdraw their request for such Demand
Registration and it shall not count as a Demand Registration.

    (b)  Anything in this Agreement to the contrary notwithstanding, it is
understood and agreed that the Company shall not be required to keep any shelf
registration effective or useable for offers and sales of the Registrable
Securities, file a post effective amendment to a shelf registration statement or
prospectus supplement or to supplement or amend any registration statement, if
the Company is then involved in discussions concerning, or otherwise engaged in,
any material financing or investment, acquisition or divestiture transaction or
other material business purpose if the Company determines in good faith that the
making of such a filing, supplement or amendment at such time would interfere
with such transaction or purpose. The Company shall promptly give the Holders of
Registrable Securities written notice of such postponement containing a general
statement of the reasons for such postponement and an approximation of the
anticipated delay. Upon receipt by a Holder of Registrable Securities of notice
of an event of the kind described in this Section 3.01(b), such Holder shall
forthwith discontinue such Holder's disposition of Registrable Securities until
such Holder's receipt of notice from the Company that such disposition may
continue and of any supplemented or amended prospectus indicated in such notice.
The Company shall use its reasonable best efforts to permit sales of Registrable
Securities on such shelf registration statement for at least 120 days during any
360-day period. In the event the Company shall give notice of an event of the
kind described in this Section 3.01 (b), the Company shall extend the period
during which the

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applicable registration statement shall be maintained effective as provided in
Section 3.01 (a) hereof by the number of days during the period from and
including the date of the giving of such notice to the date when the Company
shall give notice to the Selling Holders that such dispositions of such
Registrable Securities may continue and shall have made available to the Selling
Holders any such supplemented or amended prospectus.

    (c)  The Company will, if requested, prior to filing such registration
statement or any amendment or supplement thereto, furnish to the Selling
Holders, and each applicable managing Underwriter, if any, copies thereof, and
thereafter furnish to the Selling Holders and each such Underwriter, if any,
such number of copies of such registration statement, amendment and supplement
thereto (in each case including all exhibits thereto and documents incorporated
by reference therein) and the prospectus included in such registration statement
(including each preliminary prospectus) as the Selling Holders or each such
Underwriter may reasonably request in order to facilitate the sale of the
Registrable Securities by the Selling Holders.

    (d)  After the filing of the registration statement, the Company will
promptly notify the Selling Holders of any stop order issued or, to the
Company's knowledge, threatened to be issued by the Commission and take all
reasonable actions required to prevent the entry of such stop order or to remove
it if entered.

    (e)  The Company will use its commercially reasonable efforts to qualify the
Registrable Securities for offer and sale under such other securities or blue
sky laws of such jurisdictions in the United States as the Selling Holders
reasonably request; keep each such registration or qualification (or exemption
therefrom) effective during the period in which such registration statement is
required to be kept effective; and do any and all other acts and things which
may be reasonably necessary or advisable to enable each Selling Holder to
consummate the disposition of the Registrable Securities owned by such Selling
Holder in such jurisdictions; provided that the Company will not be required to
(i) qualify generally to do business in any jurisdiction where it would not
otherwise be required to qualify but for this paragraph 3.01(e), (ii) subject
itself to taxation in any such jurisdiction or (iii) consent to general service
of process in any such jurisdiction.

    (f)  The Company will as promptly as is practicable notify the Selling
Holders, at any time when a prospectus relating to the sale of the Registrable
Securities is required by law to be delivered in connection with sales by an
Underwriter or dealer, of the occurrence of any event requiring the preparation
of a supplement or amendment to such prospectus so that, as thereafter delivered
to the purchasers of such Registrable Securities, such prospectus will not
contain an untrue statement of a material fact or omit to state any material
fact required to be stated therein or necessary to make the statements therein,
in the light of the circumstances under which they were made, not misleading and
promptly make available to the Selling Holders and to the Underwriters any such
supplement or amendment. Upon receipt of any notice of the occurrence of any
event of the kind described in the preceding sentence, Selling Holders will
forthwith discontinue the offer and sale of Registrable Securities pursuant to
the registration statement covering such Registrable Securities until receipt by
the Selling Holders and the Underwriters of the copies of such supplemented or
amended prospectus and, if so directed by the Company, the Selling Holders will
deliver to the Company all copies, other than permanent file copies then in the
possession of Selling Holders, of the most recent prospectus covering such
Registrable Securities at the time of receipt of such notice. In the event the
Company shall give such notice,


                                       9
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the Company shall extend the period during which such registration statement
shall be maintained effective as provided in Section 3.01(a) hereof by the
number of days during the period from and including the date of the giving of
such notice to the date when the Company shall make available to the Selling
Holders such supplemented or amended prospectus.

    (g)  The Company will enter into customary agreements (including an
underwriting agreement in customary form) and take such other actions as are
required in order to expedite or facilitate the sale of such Registrable
Securities.

    (h)  At the request of any Underwriter in connection with an underwritten
offering the Company will furnish (i) an opinion of counsel, addressed to the
Underwriters, covering such customary matters as the managing Underwriter may
reasonably request and (ii) a comfort letter or comfort letters from the
Company's independent public accountants covering such customary matters as the
managing Underwriter may reasonably request.

    (i)  If requested by the managing Underwriter or any Selling Holder, the
Company shall promptly incorporate in a prospectus supplement or post effective
amendment such information as the managing Underwriter or any Selling Holder
reasonably requests to be included therein, including without limitation, with
respect to the Registrable Securities being sold by such Selling Holder, the
purchase price being paid therefor by the Underwriters and with respect to any
other terms of the underwritten offering of the Registrable Securities to be
sold in such offering, and promptly make all required filings of such prospectus
supplement or post effective amendment.

    (j)  The Company shall promptly make available for inspection by any Selling
Holder or Underwriter participating in any disposition pursuant to any
registration statement, and any attorney, accountant or other agent or
representative retained by any such Selling Holder or Underwriter (collectively,
the "Inspectors"), all financial and other records, pertinent corporate
documents and properties of the Company (collectively, the "Records"), as shall
be reasonably necessary to enable them to exercise their due diligence
responsibility, and cause the Company's officers, directors and employees to
supply all information requested by any such Inspector in connection with such
registration statement; provided, however, that unless the disclosure of such
Records is necessary to avoid or correct a misstatement or omission in the
registration statement or the release of such Records is ordered pursuant to a
subpoena or other order from a court of competent jurisdiction, the Company
shall not be required to provide any information under this subparagraph (j) if
(A) the Company believes, after consultation with counsel for the Company, that
to do so would cause the Company to forfeit an attorney-client privilege that
was applicable to such information or (B) if either (1) the Company has
requested and been granted from the Commission confidential treatment of such
information contained in any filing with the Commission or documents provided
supplementally or otherwise or (2) the Company reasonably determines in good
faith that such Records are confidential and so notifies the Inspectors in
writing unless prior to furnishing any such information with respect to (A) or
(B) such Holder of Registrable Securities requesting such information agrees to
enter into a confidentiality agreement in customary form and subject to
customary exceptions; provided further, however, that each Holder of Registrable
Securities agrees that it will, upon learning that disclosure of such Records is
sought in a court of competent jurisdiction, give notice to the Company and
allow the Company, at its expense, to undertake appropriate action and to
prevent disclosure of the Records deemed confidential.

                                       10
<PAGE>

    (k)  The Company shall cause the Registrable Securities included in any
registration statement to be (A) listed on each securities exchange, if any, on
which similar securities issued by the Company are then listed, or (B)
authorized to be quoted and/or listed (to the extent applicable) on the Nasdaq
National Market if the Registrable Securities so qualify.

    (l)  The Company shall provide a CUSIP number for the Registrable Securities
included in any registration statement not later than the effective date of such
registration statement.

    (m)  The Company shall cooperate with each Selling Holder and each
Underwriter participating in the disposition of such Registrable Securities and
their respective counsel in connection with any filings required to be made with
the National Association of Securities Dealers, Inc.

    (n)  The Company shall during the period when the prospectus is required to
be delivered under the Securities Act, promptly file all documents required to
be filed with the Commission pursuant to Sections 13(a), 13(c), 14 or 15(d) of
the Exchange Act.

    (o)  The Company will make generally available to its security holders, as
soon as reasonably practicable, an earnings statement covering a period of 12
months, beginning within three months after the effective date of the
registration statement, which earnings statement shall satisfy the provisions of
Section 11(a) of the Securities Act and the rules and regulations of the
Commission thereunder.

         The Company may require Selling Holders promptly to furnish in writing
to the Company such information regarding such Selling Holders, the plan of
distribution of the Registrable Securities and other information as the Company
may from time to time reasonably request or as may be legally required in
connection with such registration.

    SECTION 3.02. Registration Expenses. In connection with any Registration
effected hereunder, the Company shall pay the following expenses incurred in
connection with such registration (the "Registration Expenses"): (i)
registration and filing fees with the Commission and the National Association of
Securities Dealers, Inc., (ii) fees and expenses of compliance with securities
or blue sky laws (including reasonable fees and disbursements of counsel in
connection with blue sky qualifications of the Registrable Securities), (iii)
printing expenses, (iv) fees and expenses incurred in connection with the
listing or quotation of the Registrable Securities, (v) fees and expenses of
counsel to the Company and the reasonable fees and expenses of independent
certified public accountants for the Company (including fees and expenses
associated with the special audits or the delivery of comfort letters), (vi) the
reasonable fees and expenses of any additional experts retained by the Company
in connection with such registration, (vii) all roadshow costs and expenses not
paid by the Underwriters and (viii) the reasonable fees and expenses of one
counsel for the Selling Holders.

                                  ARTICLE IV

                       Indemnification and Contribution
                       --------------------------------

    SECTION 4.01. Indemnification by the Company. The Company agrees to
indemnify and hold harmless each Selling Holder and its Affiliates and their
respective officers, directors,


                                       11
<PAGE>

partners, stockholders, members, employees, agents and representatives and each
Person (if any) which controls a Selling Holder within the meaning of either
Section 15 of the Securities Act or Section 20 of the Exchange Act, from and
against any and all losses, claims, damages, liabilities, costs and expenses
(including reasonable attorneys' fees) caused by, arising out of, resulting from
or related to any untrue statement or alleged untrue statement of a material
fact contained or incorporated by reference in any registration statement or
prospectus relating to the Registrable Securities (as amended or supplemented if
the Company shall have furnished any amendments or supplements thereto) or any
preliminary prospectus, or caused by any omission or alleged omission to state
therein a material fact required to be stated therein or necessary to make the
statements therein not misleading, except insofar as such losses, claims,
damages or liabilities are caused by or based upon any information furnished in
writing to the Company by or on behalf of such Selling Holder expressly for use
therein or by the Selling Holder's failure to deliver a copy of the registration
statement or prospectus or any amendments or supplements thereto after the
Company has furnished the Selling Holder with copies of the same; provided,
however, that the Company shall have no obligation to indemnify under this
sentence to the extent any such losses, claims, damages or liabilities have been
finally and non-appealably determined by a court to have resulted from such
Selling Holder's willful misconduct or gross negligence. The Company also agrees
to indemnify any Underwriters of the Registrable Securities, their officers and
directors and each person who controls such Underwriters on substantially the
same basis as that of the indemnification of the Selling Holders provided in
this Section 4.01, except insofar as such losses, claims, damages or liabilities
are caused by or based upon any information furnished in writing to the Company
by or on behalf of such Underwriter expressly for use therein or by the
Underwriter's failure to deliver a copy of the registration statement or
prospectus or any amendments or supplements thereto after the Company has
furnished the Underwriter with copies of the same; provided, however, that the
Company shall have no obligation to indemnify under this sentence to the extent
any such losses, claims, damages or liabilities have been finally and non-
appealably determined by a court to have resulted from any such Underwriter's
willful misconduct or gross negligence.

    SECTION 4.02. Indemnification by Selling Holders. Each Selling Holder agrees
to indemnify and hold harmless the Company, its officers and directors, and each
Person, if any, which controls the Company within the meaning of either Section
15 of the Securities Act or Section 20 of the Exchange Act to the same extent as
the foregoing indemnity from the Company to each Selling Holder, but only with
reference to information furnished in writing by or on behalf of such Selling
Holder expressly for use in any registration statement or prospectus relating to
the Registrable Securities, or any amendment or supplement thereto, or any
preliminary prospectus. Each Selling Holder also agrees to indemnify and hold
harmless any Underwriters of the Registrable Securities, their officers and
directors and each person who controls such Underwriters on substantially the
same basis as that of the indemnification of the Company provided in this
Section 4.02, but only with reference to information furnished in writing by or
on behalf of such Selling Holder expressly for use in any registration statement
or prospectus relating to the Registrable Securities, or any amendment or
supplement thereto, or any preliminary prospectus. Each such Selling Holder's
liability under this Section 4.02 shall be limited to an amount equal to the net
proceeds (after deducting the underwriting discount and expenses) received by
such Selling Holder from the sale of such Registrable Securities by such Selling
Holder. The obligation of each Selling Holder shall be several and not joint.


                                       12
<PAGE>

    SECTION 4.03. Conduct of Indemnification Proceedings. In case any proceeding
(including any governmental investigation) shall be instituted involving any
Person in respect of which indemnity may be sought pursuant to Section 4.01 or
Section 4.02, such Person (the "Indemnified Party") shall promptly notify the
Person against whom such indemnity may be sought (the "Indemnifying Party") in
writing and the Indemnifying Party, upon the request of the Indemnified Party,
shall retain counsel reasonably satisfactory to such Indemnified Party to
represent such Indemnified Party and any others the Indemnifying Party may
designate in such proceeding and shall pay the fees and disbursements of such
counsel related to such proceeding. In any such proceeding, any Indemnified
Party shall have the right to retain its own counsel, but the fees and expenses
of such counsel shall be at the expense of such Indemnified Party unless (i) the
Indemnifying Party and the Indemnified Party shall have mutually agreed to the
retention of such counsel or (ii) the named parties to any such proceeding
(including any impleaded parties) include both the Indemnified Party and the
Indemnifying Party and, in the written opinion of counsel for the Indemnified
Party, representation of both parties by the same counsel would be inappropriate
due to actual or potential differing interests between them. It is understood
that the Indemnifying Party shall not, in connection with any proceeding or
related proceedings in the same jurisdiction, be liable for the fees and
expenses of more than one separate firm of attorneys (in addition to any local
counsel) at any time for all such Indemnified Parties, and that all such fees
and expenses shall be reimbursed as they are incurred. In the case of any such
separate firm for the Indemnified Parties, such firm shall be designated in
writing by the Indemnified Parties. The Indemnifying Party shall not be liable
for any settlement of any proceeding effected without its written consent, but
if settled with such consent (not to be unreasonably withheld), or if there be a
final judgment for the plaintiff, the Indemnifying Party shall indemnify and
hold harmless such Indemnified Parties from and against any loss or liability
(to the extent stated above) by reason of such settlement or judgment.

    SECTION 4.04. Contribution. If the indemnification provided for in this
Article IV is unavailable to an Indemnified Party in respect of any losses,
claims, damages or liabilities in respect of which indemnity is to be provided
hereunder, then each such Indemnifying Party, in lieu of indemnifying such
Indemnified Party, shall to the fullest extent permitted by law contribute to
the amount paid or payable by such Indemnified Party as a result of such losses,
claims, damages or liabilities in such proportion as is appropriate to reflect
the relative fault of such party in connection with the statements or omissions
that resulted in such losses, claims, damages or liabilities, as well as any
other relevant equitable considerations. The relative fault of the Company, a
Selling Holder and the Underwriters shall be determined by reference to, among
other things, whether the untrue or alleged untrue statement of a material fact
or the omission or alleged omission to state a material fact relates to
information supplied by such party and the parties' relative intent, knowledge,
access to information and opportunity to correct or prevent such statement or
omission.

    The Company and each Selling Holder agrees that it would not be just and
equitable if contribution pursuant to this Section 4.04 were determined by pro
rata allocation (even if the Underwriters were treated as one entity for such
purpose) or by any other method of allocation that does not take account of the
equitable considerations referred to in the immediately preceding paragraph. The
amount paid or payable by an Indemnified Party as a result of the losses,
claims, damages or liabilities referred to in the immediately preceding
paragraph shall be deemed to include, subject to the limitations set forth
above, any legal or other expenses reasonably incurred by such Indemnified Party
in connection with investigating or defending


                                       13
<PAGE>

any such action or claim. Notwithstanding the provisions of this Article IV, no
Underwriter shall be required to contribute any amount in excess of the amount
by which the total price at which the securities underwritten by it and
distributed to the public were offered to the public exceeds the amount of any
damages which such Underwriter has otherwise been required to pay by reason of
such untrue or alleged untrue statement or omission or alleged omission, and
each Selling Holder shall not be required to contribute any amount in excess of
the amount by which the net proceeds of the offering (before deducting expenses)
received by such Selling Holder exceeds the amount of any damages which such
Selling Holder has otherwise been required to pay by reason of such untrue or
alleged untrue statement or omission or alleged omission. No person guilty of
fraudulent misrepresentation (within the meaning of Section 11(f) of the
Securities Act) shall be entitled to contribution from any Person who was not
guilty of such fraudulent misrepresentation.


                                   ARTICLE V

                                 Miscellaneous
                                 -------------

    SECTION 5.01. Participation in Underwritten Registrations. No Person may
participate in any underwritten registered offering contemplated hereunder
unless such Person (a) agrees to sell its securities on the basis provided in
any underwriting arrangements approved by the Persons entitled hereunder to
approve such arrangements, (b) completes and executes all questionnaires, powers
of attorney, custody arrangements, indemnities, underwriting agreements and
other documents reasonably required under the terms of such underwriting
arrangements and this Agreement and (c) furnishes in writing to the Company such
information regarding such Person, the plan of distribution of the Registrable
Securities and other information as the Company may from time to time request or
as may be legally required in connection with such registration; provided,
however, that no such Person shall be required to make any representations or
warranties in connection with any such registration other than representations
and warranties as to (i) such Person's ownership of his or its Registrable
Securities to be sold or transferred free and clear of all liens, claims and
encumbrances, (ii) such Person's power and authority to effect such transfer and
(iii) such matters pertaining to compliance with securities laws as may be
reasonably requested; provided further, however, that the obligation of such
Person to indemnify pursuant to any such underwriting agreements shall be
several, not joint and several, among such Persons selling Registrable
Securities, and the liability of each such Person will be in proportion to, and
provided further that such liability will be limited to, the net amount received
by such Person from the sale of such Person's Registrable Securities pursuant to
such registration.

    SECTION 5.02. Rule 144. The Company covenants that it will file any reports
required to be filed by it under the Securities Act and the Exchange Act and
that it will take such further action as the Holders may reasonably request to
the extent required from time to time to enable the Holders to sell Registrable
Securities without registration under the Securities Act within the limitation
of the exemptions provided by Rule 144 under the Securities Act, as such Rule
may be amended from time to time, or any similar rule or regulation hereafter
adopted by the Commission. Upon the request of any Holder, the Company will
deliver to such Holder a written statement as to whether it has complied with
such reporting requirements.


                                       14
<PAGE>

    SECTION 5.03. Holdback Agreements. Each Holder agrees, in the event of an
underwritten offering by the Company (whether for the account of the Company or
otherwise) not to offer, sell, contract to sell or otherwise dispose of any
Registrable Securities, or any securities convertible into or exchangeable or
exercisable for such securities, including any sale pursuant to Rule 144 under
the Securities Act (except as part of such underwritten offering), during the 14
days prior to, and during the 120-day period (or such lesser period as the lead
or managing underwriters may require) beginning on, the effective date of the
registration statement for such underwritten offering (or, in the case of an
offering pursuant to an effective shelf registration statement pursuant to Rule
415, the pricing date for such underwritten offering).

    SECTION 5.04. Termination. The registration rights granted under this
Agreement will terminate on March 31, 2015, or such earlier time as there shall
no longer be any Registrable Securities; provided, however, that if all shares
of Series E Preferred Stock outstanding on such date shall not have been
redeemed in full in accordance with Section 10 of the Certificate of
Designations, this Agreement shall remain in full force and effect with respect
to the Registrable Securities until such time as the shares of Series E
Preferred Stock have been so redeemed in full.

    SECTION 5.05. Amendments, Waivers, Etc. This Agreement may not be amended,
waived or otherwise modified or terminated except by an instrument in writing
signed by the Company and the Holders of at least 50% of the Registrable
Securities then held by all the Holders, if the amendment is to be effective
against the Holders.

    SECTION 5.06. Counterparts. This Agreement may be executed in one or more
counterparts, all of which shall be considered one and the same agreement. Each
party need not sign the same counterpart.

    SECTION 5.07. Entire Agreement. This Agreement constitutes the entire
agreement and supersedes all prior agreements and understandings, both written
and oral, among the parties with respect to the subject matter hereof.

    SECTION 5.08. Governing Law. This Agreement shall be governed by, and
construed in accordance with, the laws of the State of New York regardless of
the laws that might otherwise govern under applicable principles of conflicts of
law thereof.

    SECTION 5.09. Assignment of Registration Rights. Each Holder of the
Registrable Securities may assign all or any part of its rights under this
Agreement to any person to whom such Holder sells, transfers or assigns such
Registrable Securities. In the event that the Holder shall assign its rights
pursuant to this Agreement in connection with the transfer of less than all its
Registrable Securities, the Holder shall also retain his rights with respect to
its remaining Registrable Securities.

    SECTION 5.10. Consent. The Company will use its best efforts to obtain the
consent of "Holders of 60% or more of the outstanding Registrable Securities"
(as such terms are defined in the Investors Rights Agreement ) (i) acknowledging
that the Holders of the Registrable Securities are new preferred stock investors
in the Company within the meaning of Section 3.3 of the Investors Rights
Agreement and (ii) consenting (if necessary in light of (i)) to the waiver of
Section 1.14(b) of the Investors Rights Agreement with respect to registrations
effected pursuant to this Agreement. If the Company does not obtain the
requisite consent referred to in the


                                       15
<PAGE>

preceding sentence prior to the first anniversary of the Closing Date (a
"Nonconsent Event"), the shelf registration rights set forth in Section 2.03
shall be adjusted as provided therein.


                                       16
<PAGE>

    IN WITNESS WHEREOF, the Company and each Holder has caused this Agreement
to be signed on its behalf by its officer thereunto duly authorized as of the
date first written above.

                                        RHYTHMS NETCONNECTIONS INC.

                                        By:
                                           -------------------------------------
                                             Name:
                                             Title:


                                        HMTF BRIDGE RHY, LLC
                                        HM4 RHYTHMS QUALIFIED FUND, LLC
                                        HM4 RHYTHMS PRIVATE FUND, LLC
                                        HM PG-IV RHYTHMS, LLC
                                        HM 4-SBS RHYTHMS COINVESTORS, LLC
                                        HM 4-EQ RHYTHMS COINVESTORS, LLC

                                        By:
                                           -------------------------------------
                                             Name:
                                             Title:


                [SIGNATURE PAGE FOR RHYTHMS NETCONNECTIONS INC.
                        REGISTRATION RIGHTS AGREEMENT]

                                       17
<PAGE>

                                  SCHEDULE I


<TABLE>
<CAPTION>
                                                                                   Number    Purchase Price
Purchasers                                                                        of Shares  of the Shares
----------                                                                        ---------  --------------

<S>                                                                               <C>        <C>
HMTF Bridge RHY, LLC                                                                125,000    $125,000,000
HM4 Rhythms Qualified Fund, LLC                                                     113,743     113,743,000
HM4 Rhythms Private Fund, LLC                                                           806         806,000
HM PG-IV Rhythms, LLC                                                                 6,056       6,056,000
HM 4-SBS Rhythms Coinvestors, LLC                                                     2,724       2,724,000
HM 4-EQ Rhythms Coinvestors, LLC                                                      1,671       1,671,000
</TABLE>

                                       18