<PAGE>


 As filed with the Securities and Exchange Commission on January 23, 2001.

                                                      Registration No. 333-49010
--------------------------------------------------------------------------------
--------------------------------------------------------------------------------
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549
                                 ------------

                              AMENDMENT NO. 4
                                       TO
                                    FORM S-1
                             REGISTRATION STATEMENT
                                     Under
                           THE SECURITIES ACT OF 1933
                                 ------------
                            CHORUM TECHNOLOGIES INC.
             (Exact Name of Registrant as Specified in its Charter)
                                 ------------
        Delaware                     3674                    84-1431727
     (State or Other           (Primary Standard          (I.R.S. Employer
     Jurisdiction of              Industrial           Identification Number)
    Incorporation or          Classification Code
      Organization)                 Number)

                1303 East Arapaho Road, Richardson, Texas 75081
                                 (214) 570-3500
  (Address, including zip code, and telephone number, including area code, of
                   registrant's principal executive offices)
                                 ------------
                                 Scott C. Grout
                     President and Chief Executive Officer
                            Chorum Technologies Inc.
                1303 East Arapaho Road, Richardson, Texas 75081
                                 (214) 570-3500
 (Name, address, including zip code, and telephone number, including area code,
                             of agent for service)
                                 ------------
                                   Copies to:
          Kirk E. Lundburg                        Christopher J. Ozburn
          Anthony M. Allen                        William B. Owens, Jr.
          James D. Robinett                         Melissa M. Munson
       F. Javier Vergne-Morell              Wilson Sonsini Goodrich & Rosati,
      Gunderson Dettmer Stough                  Professional Corporation
Villeneuve Franklin & Hachigian, LLP      8911 Capital of Texas Highway, Suite
     2700 Via Fortuna, Suite 300                          3350
         Austin, Texas 78746                    Austin, Texas 78759-7247
           (512) 732-8400                            (512) 338-5400
                                 ------------
        Approximate date of commencement of proposed sale to the public:
As soon as practicable after the effective date of this Registration Statement.

   If any of the securities being registered on this Form are to be offered on
a delayed or continuous basis pursuant to Rule 415 under the Securities Act of
1933, as amended, check the following box. [_]

   If this Form is filed to register additional securities for an offering
pursuant to Rule 462(b) under the Securities Act, please check the following
box and list the Securities Act registration statement number of the earlier
effective registration statement for the same offering. [_] ___________

   If this Form is a post-effective amendment filed pursuant to Rule 462(c)
under the Securities Act, check the following box and list the Securities Act
registration statement number of the earlier effective registration statement
for the same offering. [_] ___________

   If this form is a post-effective amendment filed pursuant to Rule 462(d)
under the Securities Act, check the following box and list the Securities Act
registration statement number of the earlier effective registration statement
for the same offering. [_] ___________

   If delivery of the prospectus is expected to be made pursuant to Rule 434,
please check the following box. [_]

   The Registrant hereby amends this Registration Statement on such date or
dates as may be necessary to delay its effective date until the Registrant
shall file a further amendment that specifically states that this Registration
Statement shall thereafter become effective in accordance with Section 8(a) of
the Securities Act of 1933, as amended, or until the Registration Statement
shall become effective on such date as the Securities and Exchange Commission,
acting pursuant to such Section 8(a), may determine.

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<PAGE>

                                    PART II

                     INFORMATION NOT REQUIRED IN PROSPECTUS

Item 13. Other Expenses of Issuance and Distribution

   The following table presents the costs and expenses, other than underwriting
discounts and commissions, payable by us in connection with the sale of common
stock being registered. All amounts are estimates except the SEC registration
fee and the NASD filing fees.

<TABLE>
   <S>                                                               <C>
   SEC Registration fee............................................. $   41,400
   NASD fee.........................................................     15,500
   Nasdaq National Market listing fee...............................      5,000
   Printing and engraving expenses..................................    175,000
   Legal fees and expenses..........................................    500,000
   Accounting fees and expenses.....................................    400,000
   Blue sky fees and expenses.......................................     10,000
   Custodian and transfer agent fees................................     10,000
   Miscellaneous fees and expenses..................................    243,100
                                                                     ----------
       Total........................................................ $1,400,000
                                                                     ==========
</TABLE>

Item 14. Indemnification of Directors and Officers

   Section 145 of the Delaware General Corporation Law authorizes a court to
award, or a corporation's board of directors to grant, indemnification to
directors and officers in terms sufficiently broad to permit indemnification
under limited circumstances for liabilities, including reimbursement for
expenses incurred, arising under the Securities Act of 1933, as amended (the
"Securities Act"). Our by-laws provide for mandatory indemnification of our
directors, officers and employees to the maximum extent permitted by the
Delaware General Corporation Law. Our Certificate of Incorporation provides
that, under Delaware law, our officers and directors shall not be liable for
monetary damages for breach of the officers' or directors' fiduciary duty as
officers or directors to our stockholders and us. This provision in the
Certificate of Incorporation does not eliminate the officers' or directors'
fiduciary duty, and in appropriate circumstances, equitable remedies like
injunctive or other forms of non-monetary relief will remain available under
Delaware law. In addition, each officer or director will continue to be subject
to liability for breach of the officer's or director's duty of loyalty to us
for acts or omissions not in good faith or involving intentional misconduct,
for knowing violations of law, for actions leading to improper personal benefit
to the officer or director, and for payment of dividends or approval of stock
repurchases or redemptions that are unlawful under Delaware law. The provision
also does not affect an officer's or director's responsibilities under any
other law, like the federal securities laws or state or federal environmental
laws. We have entered into indemnification agreements with our officers and
directors, a form of which is attached as Exhibit 10.1 and incorporated by
reference. The indemnification agreements provide our officers and directors
with further indemnification to the maximum extent permitted by the Delaware
General Corporation Law. Reference is made to Section 7 of the underwriting
agreement contained in Exhibit 1.1 to this prospectus, indemnifying officers
and directors of ours against limited liabilities.

Item 15. Recent Sales of Unregistered Securities

   Since inception, we have issued and sold the following securities:

     1. From inception through September 30, 2000, we granted direct
  issuances or stock options to purchase 13,332,215 shares of our common
  stock at exercise prices ranging from $0.03 to $3.68 per share to
  employees, consultants, directors and other service providers under our
  1997 Stock Plan.

     2. From inception through September 30, 2000, we issued and sold an
  aggregate of 7,558,681 shares of our common stock to employees,
  consultants, and other service providers for aggregate consideration of
  approximately $1,483,024 under direct issuances or exercises of options
  granted under our 1997 Stock Plan.

                                      II-1
<PAGE>

     3. In September 1997, we issued and sold 825,000 shares of our Common
  Stock for an aggregate cash purchase price of approximately $13,750 to ten
  of our employees pursuant to a series of stock purchase agreements and
  11,160,000 shares of our Series A Preferred Stock in exchange for all the
  membership interests in MacroVision Communications, L.L.C., valued at
  approximately $1,860,000 to Pei-Lan Wang, Jian-Yu Liu, Kuang-Yi Wu, Cheng
  Sun Lan, Shen Chia Wong and Li Hung Yang pursuant to a series of stock
  purchase agreements.

     4. In October 1997, we issued and sold 6,212,259 shares of our Series B-
  1 Preferred Stock for an aggregate purchase price of approximately
  $2,071,000 to a group of 19 accredited investors under a stock purchase
  agreement.

     5. In June 1998, we issued and sold 9,732,510 shares of our Series B-2
  Preferred Stock for an aggregate purchase price of approximately $4,866,000
  to a group of 19 accredited investors under a stock purchase agreement.

     6. In September 1998, we issued a warrant to purchase 75,385 shares of
  our Series B-2 Preferred Stock with an exercise price of $1.161 per share
  to Comdisco, Inc.

     7. In November 1998, we issued and sold 12,098,664 shares of our Series
  C Preferred Stock for an aggregate purchase price of approximately
  $17,140,000 to a group of 38 accredited investors under a stock purchase
  agreement.

     8. In January 1999, we issued a warrant to purchase 176,472 shares of
  our Series C Preferred Stock with an exercise price of $1.4166 per share to
  Comdisco, Inc.

     9. In November 1999, we issued and sold 9,093,843 shares of our Series D
  Preferred Stock for an aggregate purchase price of approximately
  $42,408,000 to a group of 62 accredited investors under a stock purchase
  agreement.

     10. In March 2000, we issued a warrant to purchase 77,088 shares of our
  Series D Preferred Stock with an exercise price of $4.67 per share to
  Comdisco, Inc.

     11. In June 2000, we issued a warrant to purchase 85,653 shares of our
  Series D Preferred Stock with an exercise price of $4.67 per share to
  Imperial Bancorp.

     12. In September and October 2000, we issued and sold 5,341,460 shares
  of our Series E Preferred Stock for an aggregate purchase price of
  approximately $92,087,000 to a group of 90 accredited investors under a
  stock purchase agreement.

   The sale of the above securities was deemed to be exempt from registration
under the Securities Act in reliance upon Section 4(2) of the Securities Act or
Regulation D promulgated thereunder, or Rule 701 promulgated under Section 3(b)
of the Securities Act as transactions by an issuer not involving any public
offering or transactions under compensation benefit plans and contracts
relating to compensation as provided under Rule 701. The recipients of
securities in each transaction represented their intentions to acquire the
securities for investment only and not with a view to or for sale in connection
with any distribution and appropriate legends were affixed to the share
certificates issued in these transactions. All recipients had adequate access,
through their relationships with us, to information about us.

   All of the securities referenced above are deemed restricted securities for
purposes of the Securities Act. No underwriters were involved in any of the
security sales referenced above.

                                      II-2
<PAGE>

Item 16. Exhibits and Financial Statement Schedules

   (a) Exhibits

<TABLE>
<CAPTION>
 Exhibit
   No.                                 Description
 -------                               -----------
 <C>      <S>
  1.1*    Form of Underwriting Agreement.

  3.1**   Restated Certificate of Incorporation of the Registrant, as amended
          to date.

  3.2**   Form of Amended and Restated Certificate of Incorporation of the
          Registrant to be effective upon the completion of the offering made
          under this Registration Statement.

  3.3**   Bylaws of the Registrant.

  3.4**   Form of Bylaws of the Registrant to be effective upon the completion
          of the offering made under this Registration Statement.

  4.1**   Reference is made to Exhibits 3.1, 3.2 and 3.3.

  4.2*    Form of Registrant's Common Stock certificate.

  4.3**   Amended and Restated Investors' Rights Agreement, dated September 5,
          2000.

  4.4**   Warrant to Purchase Stock between the Registrant and Imperial
          Bancorp, dated June 1, 2000.

  4.5**   Warrant to Purchase Stock between the Registrant and Comdisco, Inc.,
          dated September 22, 1998.

  4.6**   Warrant to Purchase Stock between Registrant and Comdisco, Inc.,
          dated January 22, 1999.

  4.7**   Warrant to Purchase Stock between Registrant and Comdisco, Inc.,
          dated March 1, 2000.

  5.1*    Opinion of Gunderson Dettmer Stough Villeneuve Franklin & Hachigian,
          LLP.

 10.1**   Form of Indemnification Agreement entered into between Registrant and
          its directors and officers.

 10.2**   1997 Stock Plan.

 10.3**   2000 Equity Incentive Plan.

 10.4**   Employee Stock Purchase Plan.

 10.5**   2000 Director Option Plan.

 10.6**   Lease between GUS-Enterprises-XII, L.L.C. and the Registrant, dated
          November 11, 1999.

 10.7**   Lease between Cardinal Collins Tech Center, Inc. and the Registrant,
          dated April 1, 1998.

 10.8**   Master Lease Agreement between Imperial Bank Equipment Leasing
          Division and the Registrant, dated July 7, 2000.

 10.9**   Master Lease Agreement between Comdisco, Inc. and the Registrant,
          dated September 22, 1998.

 10.10**  Lease between East Collins L.P. and the Registrant, dated October 10,
          2000.

 10.11+   OEM Purchase and Sale Agreement between Nortel Networks Limited and
          the Registrant, effective January 1, 2000.

 10.12**+ Amendment No. 1 to the OEM Purchase and Sale Agreement between Nortel
          Networks Limited, and the Registrant, dated October 12, 2000.

 21.1**   Subsidiary List.

 23.1**   Consent of Ernst & Young LLP, independent auditors.
</TABLE>


                                      II-3
<PAGE>

<TABLE>
<CAPTION>
 Exhibit
   No.                                Description
 -------                              -----------
 <C>     <S>
 23.2*   Consent of Gunderson Dettmer Stough Villeneuve Franklin & Hachigian,
         LLP, counsel to the Registrant. Reference is made to Exhibit 5.1.

 24.1**  Power of Attorney. Reference is made to page II-5.

 27.1**  Financial Data Schedule.

 99.1**  Consent of Ryan, Hankin & Kent
</TABLE>

---------------------
* To be filed by amendment.
**Previously filed.
+ Confidential treatment requested.

   All schedules have been omitted because the information required to be
presented in them is not applicable or is shown in the financial statements or
related notes.

Item 17. Undertakings

   We undertake to provide to the underwriters at the closing specified in the
underwriting agreement, certificates in the denominations and registered in the
names as required by the underwriters to permit prompt delivery to each
purchaser.

   Insofar as indemnification for liabilities arising under the Securities Act
of 1933 may be permitted to directors, officers and controlling persons of the
Registrant under the Delaware General Corporation Law, the Certificate of
Incorporation or our by-laws, the underwriting agreement, or otherwise, we have
been advised that in the opinion of the Securities and Exchange Commission this
indemnification is against public policy as expressed in the Securities Act,
and is, therefore, unenforceable. In the event that a claim for indemnification
against these liabilities, other than the payment by us of expenses incurred or
paid by a director, officer, or controlling person of ours in the successful
defense of any action, suit or proceeding, is asserted by a director, officer
or controlling person in connection with the securities being registered in
this offering, we will, unless in the opinion of our counsel the matter has
been settled by controlling precedent, submit to a court of appropriate
jurisdiction the question of whether this indemnification by us is against
public policy as expressed in the Securities Act and will be governed by the
final adjudication of this issue.

   We undertake that:

     (1) For purposes of determining any liability under the Securities Act,
  the information omitted from the form of prospectus filed as part of this
  registration statement in reliance upon Rule 430A and contained in a form
  of prospectus filed by us under Rule 424(b)(1) or (4) or 497(h) under the
  Securities Act shall be deemed to be part of this registration statement as
  of the time it was declared effective.

     (2) For the purpose of determining any liability under the Securities
  Act, each post-effective amendment that contains a form of prospectus shall
  be deemed to be a new registration statement relating to the securities
  offered, and the offering of these securities at that time shall be deemed
  to be the initial bona fide offering.

                                      II-4
<PAGE>

                                   SIGNATURES

   Pursuant to the requirements of the Securities Act of 1933, the Registrant
has duly caused this Amendment No. 4 to the Registration Statement to be signed
on its behalf by the undersigned, thereunto duly authorized, in the City of
Richardson, State of Texas, on this 23rd day of January, 2001.

                                          Chorum Technologies Inc.

                                                     /s/ Scott C. Grout
                                          By: _________________________________
                                                       Scott C. Grout
                                               President and Chief Executive
                                                          Officer

                               POWER OF ATTORNEY

   Pursuant to the requirements of the Securities Act of 1933, as amended, this
Amendment No. 4 to the Registration Statement has been signed by the following
persons in the capacities and on the dates indicated:

<TABLE>
<CAPTION>
              Signature                          Title                   Date
              ---------                          -----                   ----

<S>                                    <C>                        <C>
          /s/ Scott C. Grout           President (Principal        January 23, 2001
______________________________________  Executive Officer), Chief
            Scott C. Grout              Executive Officer and
                                        Director

        /s/ Jian-Yu Liu, Ph.D*         Chief Technology Officer    January 23, 2001
______________________________________  and Director
          Jian-Yu Liu, Ph.D

          /s/ S. Kent Coker*           Chief Financial Officer     January 23, 2001
______________________________________  (Principal Financial and
            S. Kent Coker               Accounting Officer) and
                                        Chief Operating Officer

        /s/ Robert J. Paluck*          Director and Chairman of    January 23, 2001
______________________________________  the Board
           Robert J. Paluck

        /s/ Philip T. Gianos*          Director                    January 23, 2001
______________________________________
           Philip T. Gianos

      /s/ Jon W. Bayless, Ph.D.*       Director                    January 23, 2001
______________________________________
        Jon W. Bayless, Ph.D.
</TABLE>



                                      II-5
<PAGE>

<TABLE>
<CAPTION>
              Signature                          Title                   Date
              ---------                          -----                   ----

<S>                                    <C>                        <C>
        /s/ Edward E. Olkkola*         Director                    January 23, 2001
______________________________________
          Edward E. Olkkola
</TABLE>


       /s/ Scott C. Grout
*By:_____________________________
         Scott C. Grout
        Attorney-in-Fact
  Pursuant to Power of Attorney

                                      II-6
<PAGE>

                               INDEX TO EXHIBITS

<TABLE>
<CAPTION>
 Exhibit
   No.                                 Description
 -------                               -----------
 <C>      <S>
  1.1*    Form of Underwriting Agreement.

  3.1**   Restated Certificate of Incorporation of the Registrant, as amended
          to date.

  3.2**   Form of Amended and Restated Certificate of Incorporation of the
          Registrant to be effective upon the completion of the offering made
          under this Registration Statement.

  3.3**   Bylaws of the Registrant.

  3.4**   Form of Bylaws of the Registrant to be effective upon the completion
          of the offering made under this Registration Statement.

  4.1**   Reference is made to Exhibits 3.1, 3.2 and 3.3.

  4.2*    Form of Registrant's Common Stock certificate.

  4.3**   Amended and Restated Investors' Rights Agreement, dated September 5,
          2000.

  4.4**   Warrant to Purchase Stock between the Registrant and Imperial
          Bancorp, dated June 1, 2000.

  4.5**   Warrant to Purchase Stock between the Registrant and Comdisco, Inc.,
          dated September 22, 1998.

  4.6**   Warrant to Purchase Stock between Registrant and Comdisco, Inc.,
          dated January 22, 1999.

  4.7**   Warrant to Purchase Stock between Registrant and Comdisco, Inc.,
          dated March 1, 2000.

  5.1*    Opinion of Gunderson Dettmer Stough Villeneuve Franklin & Hachigian,
          LLP.

 10.1**   Form of Indemnification Agreement entered into between Registrant and
          its directors and officers.

 10.2**   1997 Stock Plan.

 10.3**   2000 Equity Incentive Plan.

 10.4**   Employee Stock Purchase Plan.

 10.5**   2000 Director Option Plan.

 10.6**   Lease between GUS-Enterprises-XII, L.L.C. and the Registrant, dated
          November 11, 1999.

 10.7**   Lease between Cardinal Collins Tech Center, Inc. and the Registrant,
          dated April 1, 1998.

 10.8**   Master Lease Agreement between Imperial Bank Equipment Leasing
          Division and the Registrant, dated July 7, 2000.

 10.9**   Master Lease Agreement between Comdisco, Inc. and the Registrant,
          dated September 22, 1998.

 10.10**  Lease between East Collins L.P. and the Registrant, dated October 10,
          2000.

 10.11+   OEM Purchase and Sale Agreement between Nortel Networks Limited and
          the Registrant, effective January 1, 2000.

 10.12**+ Amendment No. 1 to the OEM Purchase and Sale Agreement between Nortel
          Networks Limited and the Registrant, dated October 12, 2000.

 21.1**   Subsidiary List.

 23.1**   Consent of Ernst & Young LLP, independent auditors.

 23.2*    Consent of Gunderson Dettmer Stough Villeneuve Franklin & Hachigian,
          LLP, counsel to the Registrant. Reference is made to Exhibit 5.1.

 24.1**   Power of Attorney. Reference is made to page II-5.

 27.1**   Financial Data Schedule.

 99.1**   Consent of Ryan, Hankin & Kent
</TABLE>
---------------------
* To be filed by amendment.
** Previously filed.
+ Confidential treatment requested.

                                      II-7