EXHIBIT 4.15

                         REGISTRATION RIGHTS AGREEMENT


      REGISTRATION RIGHTS AGREEMENT, dated as of December 31, 1997 (the
"Agreement"), by and between CASTLE DENTAL CENTERS, INC., a Delaware corporation
(the "Company"), and STEVE W. LEBO, D.D.S. (the "Holder").

      1. INTRODUCTIONContemporaneously with the execution and delivery of this
Agreement, the Holder acquired an aggregate of 165,000 shares of the Company's
Common Stock $.001 par value per share ("Common Stock") on December 8, 1997,
subject to adjustment for stock splits, stock dividends, reclassification and
similar events. The Company has agreed to grant to the Holder and hereby grants
under this Agreement certain registration rights with respect to the shares of
Common Stock held by the Holder. Prior to the date hereof, the Company entered
into a Registration Rights Agreement dated as of December 18, 1995, as amended
and restated on June 16, 1997, with purchasers of the Company's Series A and
Series C Convertible Preferred Stock and certain other parties, as may be
amended from time to time (the "Original Registration Rights Agreement"), and
other agreements regarding registration of Common Stock with other parties.
Certain terms used herein which are not otherwise defined are defined in the
Original Registration Rights Agreement.

      2.    REGISTRATION UNDER SECURITIES ACT.

            2.1   "PIGGYBACK" REGISTRATIONS.

                  (a) RIGHT TO INCLUDE HOLDER SECURITIES. If the Company at any
time proposes to register any Common Stock under the Securities Act (other than
by a registration on Form S-4, Form S-8 or any successor or similar form, or in
connection with a tender offer, merger, or other acquisition), for sale for its
own account, and such Common Stock is to be distributed by or through one or
more underwriters on a firm commitment basis, it will at such time give prompt
written notice to all Holders of Holder Securities (as hereinafter defined) of
its intention to do so and of such Holders' rights under this Section 2.1. Upon
the written request of any such Holder made within 20 days after the date of any
such notice given in accordance with Section 7 hereof, the Company will use its
reasonable best efforts to effect the registration under the Securities Act of
all Holder Securities which the Company has been so requested to register by the
Holders thereof, and to arrange for such underwriters to include all the Holder
Securities to be offered and sold by such Holder among the Common Stock to be
distributed by such underwriters, PROVIDED that if, at any time after giving
written notice of its intention to register its Common Stock and prior to the
effective date of the registration statement filed in connection with such
registration, the Company shall determine for any reason not to register or to
delay registration of its Common Stock, the Company may, at its election, give
written notice of such determination to each Holder of Holder Securities and,
thereupon, (i) in the case of a determination not to register, shall be relieved
of its obligation to register any Holder Securities in connection with such
registration (but not from its
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obligation to pay the Registration Expenses in connection therewith or to
include any Holder Securities in subsequent registrations), and (ii) in the case
of a determination to delay registering, shall be permitted to delay registering
any Holder Securities for the same period as the delay in registering its Common
Stock. The Holders of Holder Securities to be distributed by such underwriters
shall be parties to the underwriting agreement between the Company and such
underwriters. Any such Holder of Holder Securities shall not be required to make
any representations or warranties to or agreements with the Company or the
underwriters other than representations, warranties, or agreements typical in an
offering of this type, including those regarding such Holder, such Holder's
Holder Securities and such Holder's intended method of distribution, any other
information supplied by such Holder to the Company for use in the registration
statement and any other representation required by law. The Company will pay all
Registration Expenses in connection with each registration of Holder Securities
requested pursuant to this Section 2.1.

                  (b) APPORTIONMENT IN "PIGGYBACK" REGISTRATIONS. If the
managing underwriter of such underwritten offering shall inform the Company and
the Holders of the Holder Securities requesting such registration in writing of
its belief that the aggregate number of shares of Common Stock requested to be
included in such registration (including any securities of other securityholders
of the Company included in such registration pursuant to the terms of the
Original Registration Rights Agreement) exceeds the number which can be sold in
(or during the time of) such offering or that the inclusion would adversely
affect the marketing or the selling price of the Common Stock to be sold by the
Company therein, then the Company may include all securities proposed by the
Company to be sold for its own account and may decrease or eliminate the number
of Holder Securities requested to be included in such registration to the extent
necessary to reduce the number of shares of Common Stock to be included in the
registration to the level recommended by the managing underwriter. In the event
that such a reduction is necessary, the number of Holder Securities to be
included in such registration shall be reduced, on a pro rata basis among
Holders (based on the total number of shares of Common Stock owned by Holders
and other parties (but excluding for purposes of this calculation Common Stock
which constitutes Registrable Inside Shareholder Securities and Registrable
Securities) and requested to be included in such registration), prior to any
reduction in the number of Registrable Inside Shareholder Securities and
Registrable Securities to be included in such registration.

            2.2 REGISTRATION PROCEDURES. If and whenever the Company is required
to use its reasonable best efforts to effect the registration of any Holder
Securities under the Securities Act as provided in Section 2.1, the Company will
as expeditiously as possible:

                        (i) prepare and file with the Commission the requisite
      registration statement to effect such registration and thereafter use its
      reasonable best efforts to cause such registration statement to become
      effective, PROVIDED that the Company may discontinue

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      any registration of its securities at any time prior to the effective date
      of the registration statement relating thereto;

                        (ii) prepare and file with the Commission such
      amendments and supplements to such registration statement and the
      prospectus used in connection therewith as may be necessary to keep such
      registration statement effective and to comply with the provisions of the
      Securities Act with respect to the disposition of all securities covered
      by such registration statement until such time as all of such securities
      have been disposed of in accordance with the intended methods of
      disposition by the seller or sellers thereof set forth in such
      registration statement or for six months, whichever period is shorter;

                        (iii) furnish to each seller of Holder Securities
      covered by such registration statement such number of conformed copies of
      such registration statement and of each such amendment and supplement
      thereto, such number of copies of the prospectus contained in such
      registration statement (including each preliminary prospectus and any
      summary prospectus) and any other prospectus filed under Rule 424 or Rule
      430A under the Securities Act, in conformity with the requirements of the
      Securities Act, and such other documents, as such seller may reasonably
      request;

                        (iv) use its best efforts to register or qualify all
      Holder Securities covered by such registration statement under such other
      securities or blue sky laws of such jurisdictions as each seller thereof
      shall reasonably request, to keep such registration or qualification in
      effect for so long as such registration statement remains in effect, and
      take any other action which may be reasonably necessary to enable such
      seller to consummate the disposition in such jurisdictions of the
      securities owned by such seller, except that the Company shall not for any
      such purpose be required to qualify generally to do business as a foreign
      corporation in any jurisdiction wherein it would not but for the
      requirements of this subdivision (iv) be obligated to be so qualified or
      to consent to general service of process in any such jurisdiction or
      subject itself to be required to pay any franchise or income taxes in any
      such jurisdiction;

                        (v) use its reasonable best efforts to cause all Holder
      Securities covered by such registration statement to be registered with or
      approved by such other governmental agencies or authorities as may be
      necessary to enable the seller or sellers thereof to consummate the
      disposition of such Holder Securities;

                        (vi) furnish to each seller of Holder Securities a
      signed counterpart, addressed to such seller, of:

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                              (x) an opinion of counsel for the Company, dated
            the date of the closing under the underwriting agreement, reasonably
            satisfactory in form and substance to such underwriter, and

                              (y) a "comfort" letter, dated the effective date
            of such registration statement and dated the date of the closing
            under the underwriting agreement, signed by the independent public
            accountants who have certified the Company's financial statements
            included in such registration statement, addressed to each seller,
            to the extent the same can be reasonably obtained, and addressed to
            the underwriters, covering substantially the same matters with
            respect to such registration statement (and the prospectus included
            therein) and, in the case of the accountants' letter, with respect
            to events subsequent to the date of such financial statements, as
            are customarily covered in accountants' letters delivered to the
            underwriters in underwritten public offerings of securities and such
            other financial matters as such seller or the underwriters may
            reasonably request;

                        (vii) notify each seller of Holder Securities covered by
      such registration statement, at any time when a prospectus relating
      thereto is required to be delivered under the Securities Act, upon
      discovery that, or upon the happening of any event as a result of which,
      the prospectus included in such registration statement, as then in effect,
      includes an untrue statement of a material fact or omits to state any
      material fact required to be stated therein or necessary to make the
      statements therein not misleading in the light of the circumstances under
      which they were made, and at the request of any such seller or Holder
      promptly prepare to furnish to such seller or Holder a reasonable number
      of copies of a supplement to or an amendment of such prospectus as may be
      necessary so that, as thereafter delivered to the sellers of such
      securities, such prospectus shall not include an 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 not misleading in the
      light of the circumstances under which they were made;

                        (viii)otherwise use its reasonable best efforts to
      comply with all applicable rules and regulations of the Commission, and
      make available to its security holders, as soon as reasonably practicable,
      an earnings statement covering the period of at least 12 months, but not
      more than 18 months, beginning with the first full calendar month after
      the effective date of such registration statement, which earnings
      statement shall satisfy the provisions of Section ll(a) of the Securities
      Act;

                        (ix) provide and cause to be maintained a transfer agent
      and registrar for all Holder Securities covered by such registration
      statement from and after a date not later than the effective date of such
      registration statement; and

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                        (x) use its best efforts to list all Holder Securities
      covered by such registration statement on any securities exchange on which
      any of the Company's Common Stock is then listed.

            The Company may require each proposed seller of Holder Securities as
to which any registration is being effected to promptly furnish the Company, as
a condition precedent to including such Holder's Holder Securities in any
registration, such information regarding such seller and the distribution of
such securities as the Company may from time to time reasonably request in
writing.

            Each Holder of Holder Securities agrees by acquisition of such
Holder Securities that upon receipt of any notice from the Company of the
happening of any event of the kind described in subdivision (vii) of this
Section 2.2, such Holder will forthwith discontinue its disposition of Holder
Securities pursuant to the registration statement relating to such Holder
Securities until such Holder's receipt of the copies of the supplemented or
amended prospectus contemplated by subdivision (vii) of this Section 2.2 and, if
so directed by the Company, will deliver to the Company (at the Company's
expense) all copies, other than permanent file copies, then in such Holder's
possession of the prospectus relating to such Holder Securities current at the
time of receipt of such notice.

            2.3 PREPARATION; REASONABLE INVESTIGATION. In connection with the
preparation and filing of each registration statement under the Securities Act
pursuant to this Agreement, the Company will give the Holders of Holder
Securities registered under such registration statement, their underwriters, if
any, and their respective counsel (such counsel representing Holders to be
appointed by the holders of more than 50% by number of shares of Common Stock
being registered other than shares being registered by the Company) the
opportunity to participate in the preparation of such registration statement,
each prospectus included therein or filed with the Commission, and each
amendment thereof or supplement thereto, and will give each of them such access
to its books and records and such opportunities to discuss the business of the
Company with its officers and the independent public accountants who have
certified its financial statements as shall be necessary, in the opinion of such
Holders' and such underwriters' respective counsel, to conduct a reasonable
investigation within the meaning of the Securities Act.

            2.4   INDEMNIFICATION.

                  (a) INDEMNIFICATION BY THE COMPANY. In the event of any
registration of any securities of the Company under the Securities Act, the
Company will indemnify and hold harmless the seller of any Holder Securities
covered by such registration statement, its directors and officers, and each
other Person, if any, who controls such seller within the meaning of the
Securities Act, against any losses, claims, damages or liabilities, joint or
several, to which such seller or any such director or officer or controlling
person may become subject under the Securities Act or

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otherwise, insofar as such losses, claims, damages or liabilities (or actions or
proceedings, whether commenced or threatened, in respect thereof) arise out of
or are based upon any untrue statement or alleged untrue statement of any
material fact contained in any registration statement under which such
securities were registered under the Securities Act, any preliminary prospectus,
final prospectus or summary prospectus contained therein, or any amendment or
supplement thereto, or 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, and the Company will reimburse such seller and each such
director, officer and controlling person for any legal or any other expenses
reasonably incurred by them in connection with investigating or defending any
such loss, claim, liability, action or proceeding; PROVIDED, HOWEVER, that the
Company will not be liable in any such case to the extent that any such loss,
claim, damage or liability arises out of or is based upon (a) an untrue
statement or alleged untrue statement or omission or alleged omission made in
such registration statement, said preliminary or final prospectus or said
amendment or supplement in reliance upon and in conformity with written
information furnished to the Company by an instrument duly executed by such
seller, specifically for use in the preparation thereof or (b) an untrue
statement or alleged untrue statement, omission or alleged omission in a
prospectus if such untrue statement or alleged untrue statement, omission or
alleged omission is corrected in an amendment or supplement to the prospectus or
in the final prospectus, which amendment, supplement or final prospectus is
delivered to such seller and such seller thereafter fails to deliver such
prospectus as so amended or supplemented prior to or concurrently with the sale
of registered Holder Securities to the person asserting such loss, claim,
damage, liability or expense. Such indemnity shall remain in full force and
effect regardless of any investigation made by or on behalf of such seller or
any such director, officer or controlling person and shall survive the transfer
of such securities by such seller.

                  (b) INDEMNIFICATION BY THE HOLDERS. The Holders will, and
hereby do, indemnify and hold harmless (in the same manner and to the same
extent as set forth in subdivision (a) of this Section 2.4) the Company, each
director of the Company, each officer of the Company and each other Person, if
any, who controls the Company within the meaning of the Securities Act with
respect to any statement or alleged statement in or omission or alleged omission
from such registration statement, any preliminary prospectus, final prospectus
or summary prospectus contained therein, or any amendment or supplement thereto,
if such statement or alleged statement or omission or alleged omission was made
in reliance upon and in conformity with written information furnished to the
Company by such Holder for use in the preparation of such registration
statement, preliminary prospectus, final prospectus, summary prospectus,
amendment or supplement. Such indemnity shall remain in full force and effect,
regardless of any investigation made by or on behalf of the Company or any such
director, officer or controlling Person and shall survive the transfer of such
securities by such Holder.

                  (c) NOTICE. Promptly after receipt by an indemnified party of
notice of the commencement of any action or proceeding involving a claim
referred to in the preceding

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subdivisions of this Section 2.4, such indemnified party will, if a claim in
respect thereof is to be made against an indemnifying party, give written notice
to the latter of the commencement of such action, PROVIDED that the failure of
any indemnified party to give notice as provided herein shall not relieve the
indemnifying party of its obligations under the preceding subdivisions of this
Section 2.4, except to the extent that the indemnifying party is prejudiced by
such failure to give notice. In case any such action is brought against an
indemnified party, unless in such indemnified party's reasonable judgment a
conflict of interest between such indemnified party and indemnifying parties may
exist in respect of such claim, the indemnifying party shall be entitled to
participate in and to assume the defense thereof, jointly with any other
indemnifying party similarly notified to the extent that it may wish, with
counsel reasonably satisfactory to such indemnified party, and after notice from
the indemnifying party to such indemnified party of its election so to assume
the defense thereof, the indemnifying party shall not be liable to such
indemnified party for any legal or other expenses subsequently incurred by the
latter in connection with the defense thereof other than reasonable costs of
investigation. No indemnifying party shall, without the consent of the
indemnified party, consent to entry of any judgment or enter into any settlement
which does not include as an unconditional term thereof the giving by the
claimant or plaintiff to such indemnified party of a release from all liability
in respect to such claim or litigation.

                  (d) INDEMNIFICATION PAYMENTS. The indemnification required by
this Section 2.4 shall be made by periodic payments of the amount thereof during
the course of the investigation or defense, as and when bills are received or
expense, loss, damage or liability is incurred.

            2.5 ADJUSTMENTS AFFECTING HOLDER SECURITIES. The Company will not
effect or permit to occur any combination or subdivision of shares which would
adversely affect the ability of the holders of Holder Securities to include such
Holder Securities in any registration of its securities contemplated by this
Section 2 or the marketability of such Holder Securities under any such
registration.

      3. DEFINITIONS. As used herein, unless the context otherwise requires, the
following terms have the following respective meanings:

COMMISSION: The Securities and Exchange Commission or any other federal agency
at the time administering the Securities Act.

COMMON STOCK: Common Stock of the Company, $.001 par value per share, and stock
of any other class with which such shares may hereafter have been exchanged or
reclassified.

EXCHANGE ACT: The Securities Exchange Act of 1934, as amended, and the rules and
regulations promulgated thereunder.

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HOLDER SECURITIES: The Common Stock (a) issued to the Holders on the date of
this Agreement and (b) issued to the Holders on conversion of the Preferred
Stock and any securities issued or issuable with respect to such Common Stock by
way of stock dividend or stock split or in connection with a combination of
shares, recapitalization, merger, consolidation or other reorganization or
otherwise upon any required adjustments.

As to any particular Holder Securities, such securities shall cease to be Holder
Securities when (a) 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 in accordance with such registration
statement, (b) they shall have been distributed to the public pursuant to Rule
144 (or any successor provision) under the Securities Act, (c) they are eligible
for distribution to the public under Rule 144(k), or (d) they shall have ceased
to be outstanding.

PERSON: A corporation, an association, a partnership, a business or other
business entity, an individual, a governmental or political sub-division thereof
or a governmental agency.

REGISTRABLE INSIDE SHAREHOLDER SECURITIES: Shall have the meaning assigned to
such term in the Original Registration Rights Agreement.

REGISTRABLE SECURITIES: Shall have the meaning assigned to such term in the
Original Registration Rights Agreement.

REGISTRATION EXPENSES: All expenses incident to the Company's performance of or
compliance with Section 2, including, without limitation, all registration,
filing and National Association of Securities Dealers, Inc. fees, all fees and
expenses of complying with securities or blue sky laws, all word processing,
duplicating and printing expenses, messenger and delivery expenses, the
reasonable fees and disbursements of counsel for the Company and of its
independent public accountants, including the expenses of any special audits or
"comfort" letters required by or incident to such performance and compliance,
the reasonable fees and disbursements of one counsel (except in the event of a
conflict of interest, then such number of counsel as is appropriate to resolve
such conflict) retained by the holder or holders of more than 50% by number of
shares of Common Stock being registered other than shares registered by the
Company, premiums and other costs of policies of insurance obtained by the
Company against liabilities arising out of the public offering of the
Registrable Securities and the Holder Securities being registered and any fees
and disbursements of underwriters customarily paid by issuers or sellers of
securities, including reasonable fees of underwriters counsel including
qualification of securities under blue sky laws, but excluding all agency fees
and commissions, underwriting discounts and commissions and transfer taxes, if
any.

SECURITIES ACT: The Securities Act of 1933, as amended, and the rules and
regulations promulgated thereunder.

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      4. RULE 144. If the Company shall have filed a registration statement
pursuant to the requirements of Section 12 of the Exchange Act or a registration
statement pursuant to the requirements of the Securities Act, the Company will
file the reports required to be filed by it, and in the manner required to be
filed by it, under the Securities Act and the Exchange Act (or, if the Company
is not required to file such reports, will, upon the request of any Holder of
Holder Securities, make publicly available other information) and will take such
further action as any Holder of Holder Securities may reasonably request, all to
the extent required from time to time to enable such Holder to sell Holder
Securities without registration under the Securities Act within the limitation
of the exemptions provided by (a) Rule 144 under the Securities Act, as such
rule may be amended from time to time or (b) any similar rule or regulation
hereafter adopted by the Commission ("Rule 144"). Upon the request of any Holder
of Holder Securities, the Company will deliver to such Holder a written
statement as to whether it has complied with such requirements.

      5. AMENDMENTS AND WAIVERS. This Agreement may be amended and the Company
may take any action herein prohibited or omit to perform any act herein required
to be performed by it, only if the Company shall have obtained the written
consent to such amendment, action or omission to act, of the holder or holders
of 662/3% or more (by number of shares) of Holder Securities. Each holder of any
Holder Securities at the time or thereafter outstanding shall be bound by any
consent authorized by this Section 5, whether or not such Holder Securities
shall have been marked to indicate such consent.

      6. NOMINEES FOR BENEFICIAL OWNERS. In the event that any Holder Securities
are held by a nominee for the beneficial owner thereof, the beneficial owner
thereof may upon the giving of written notice to the Company, at its election,
be treated as the Holder of such Holder Securities for purposes of any request
or other action by any Holder or Holders of Holder Securities pursuant to this
Agreement or any determination of any number or percentage of shares of Holder
Securities held by any Holder or Holders of Holder Securities contemplated by
this Agreement. The Company may require assurances reasonably satisfactory to it
of such owner's beneficial ownership of such Holder Securities.

      7. NOTICES. All communications provided for hereunder shall be sent by
first-class mail or overnight courier and (a) if addressed to a party other than
the Company, addressed to such party at the address set forth on the signature
page hereof, or at such other address as such party shall have furnished to the
Company in writing, or (b) if addressed to any other Holder of Holder
Securities, at the address that such Holder shall have furnished to the Company
in writing, or, until any such other holder so furnishes to the Company an
address, then to and at the address of the last holder of such Holder Securities
who has furnished an address to the Company, or (c) if addressed to the Company,
at 1360 Post Oak Boulevard, Suite 1300, Houston, Texas 77056, to the attention
of its President, or to the attention of such other officer, as the Company
shall have furnished to each Holder of Securities at the time outstanding.

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      8. ASSIGNMENT. This Agreement shall be binding upon and inure to the
benefit of and be enforceable by the parties hereto and their respective
successors and assigns. In addition, and whether or not any express assignment
shall have been made, the provisions of this Agreement which are for the benefit
of the parties hereto other than the Company shall also be for the benefit of
and enforceable by any subsequent Holder of any Holder Securities.

      9. DESCRIPTIVE HEADINGS. The descriptive headings of the several sections
and paragraphs of this Agreement are inserted for reference only and shall not
limit or otherwise affect the meaning hereof.

      10. GOVERNING LAW. This agreement shall be construed and enforced in
accordance with, and the rights of the parties shall be governed by, the laws of
the state of Delaware.

      11. COUNTERPARTS. This Agreement may be executed simultaneously in any
number of counterparts, each of which shall be deemed an original, but all such
counterparts shall together constitute one and the same instrument.

      12. EXTENSION OF ADDITIONAL REGISTRATION RIGHTS. The Holders acknowledge
that the Company retains the right to extend the same or similar registration
rights to other holders of its securities, and that such action by the Company
shall not constitute a breach of or a default under this Agreement. It is the
intent of the parties hereto that this Agreement be read consistently with the
Original Registration Rights Agreement, but in the event of a conflict
therewith, the provisions of the Original Registration Rights Agreement shall
control. If the Company shall hereafter grant any demand registration rights
with respect to the securities of the Company that are more favorable than the
rights granted under this Agreement, the Holder shall immediately be vested with
such more favorable rights.

      13. LOCK-UP AGREEMENT. If requested in writing by the underwriters for any
underwritten public offering of securities of the Company, the Holders shall
agree not to sell publicly any shares of Common Stock (other than Common Stock
being registered in such offering), without the consent of such underwriters,
for a period of not more than 270 days following the effective date of the
registration statement relating to such offering.

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      IN WITNESS WHEREOF, the parties have caused this Agreement to be executed
and delivered by their respective officers thereunto duly authorized as of the
date first above written.

                                       CASTLE DENTAL CENTERS, INC.


                                        By: JACK H. CASTLE, JR.
                                            Jack H. Castle, Jr.
                                            Chairman and Chief Executive Officer


ADDRESS:                                  HOLDER:


7505 Glenview, Suite 1                  By:STEVE W. LEBO, D.D.S.
Tarrant, Texas 76180                       Steve W. Lebo, D.D.S.

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