Exhibit 5.1



                               February 22, 1999



The Board of Directors
CAIS Internet, Inc.
1255 22nd Street, N.W.
Fourth Floor
Washington, D.C. 20037

     Re:  Registration Statement on Form S-1

Gentlemen:

     We have acted as counsel to CAIS Internet, Inc., a Delaware corporation
(the "Company"), with respect to the Company's Registration Statement on 
Form S-1 (the "Registration Statement"), filed with the Securities and Exchange
Commission, in connection with the registration under the Securities Act of
1933, as amended (the "Securities Act"), of up to ______ shares of the Company's
common stock, par value $.01 per share ("Common Shares"). This opinion letter is
furnished to you at your request to enable you to fulfill the requirements of
Item 601(b)(5) of Regulation S-K, 17 C.F.R. (S) 229.601(b)(5), in connection
with the Registration Statement.

     As counsel to the Company, we have examined the Company's Certificate of
Incorporation, as amended (the "Certificates"), and such Company records,
certificates and other documents and relevant statutes, regulations, published
rulings and such questions of law as we considered necessary or appropriate for
the purpose of this opinion.

     In our examination, we have assumed the authenticity of original documents,
the accuracy of copies and the genuineness of signatures.  We have relied upon
the representations and statements of officers and other representatives of the
Company with respect to the factual determinations underlying the legal
conclusions set forth herein.  We have not attempted to verify independently
such representations and statements.

     This opinion letter is based as to matters of law solely on the General
Corporation Law of the State of Delaware as currently in effect, and we express
no opinion herein as to any other laws, statutes, ordinances, rules or
regulations.

     Based upon, subject to and limited by the foregoing and the other
qualifications herein, we are of the opinion that, when the Registration
Statement has become effective under the Securities Act, upon issuance and
delivery of such Common Shares against payment of valid consideration 

 
The Board of Directors
February 22, 1999
Page 2

therefor as contemplated by the Registration Statement, such Common Shares will
be validly issued, fully paid and nonassessable.

     We hereby consent to the filing of this opinion as an exhibit to the
Registration Statement and to all references to our firm in the Registration
Statement.  In giving this consent, we do not admit that we come within the
category of persons whose consent is required under Section 7 of the Securities
Act or the rules promulgated thereunder.

     This opinion is rendered solely for your benefit in connection with the
transactions described above upon the understanding that we are not hereby
assuming any professional responsibility to any other person.  Except as
provided in the preceding paragraph, this opinion may not be relied upon by any
other person and this opinion may not be used, disclosed, quoted, filed with a
governmental agency or otherwise referred to without our express prior written
consent.  The opinions expressed in this letter are limited to the matters
expressly set forth herein, and no other opinions should be inferred beyond the
matters expressly stated herein.


                               Very truly yours,


                               SWIDLER BERLIN SHEREFF FRIEDMAN, LLP