Via Facsimile and U.S. Mail
Mail Stop 6010


February 7, 2006


Mr. Vikas Sinha
Senior Vice President and
Chief Financial Officer
Alexion Pharmaceuticals, Inc.
352 Knotter Drive
Cheshire, CT 06410


Re:	Alexion Pharmaceuticals, Inc
	Form 10-K for the Fiscal Period Ended July 31, 2005
	File No. 000-27756


Dear Mr. Sinha:

      We have reviewed your filing and have the following comment.
We have limited our review to only your financial statements and
related disclosures and do not intend to expand our review to
other
portions of your document.  In our comment, we ask you to provide
us
with information so we may better understand your disclosure.
Please
be as detailed as necessary in your explanation.  After reviewing
this information, we may raise additional comments.

	Please understand that the purpose of our review process is
to
assist you in your compliance with the applicable disclosure
requirements and to enhance the overall disclosure in your filing.
We look forward to working with you in these respects.  We welcome
any questions you may have about our comments or any other aspect
of
our review.  Feel free to call us at the telephone numbers listed
at
the end of this letter.


Form 10-K - July 31, 2005

Financial Statements

2.  Collaborations, page F-12

Procter & Gamble Pharmaceuticals Collaboration, page F-12

1. Your disclosure under "Research and Development Expenses" on
page
F-9 states that you net costs reimbursed under collaborator
agreements against your research and development costs.  Your Form
10-Q for the period ended October 31, 2005 indicates that you are
currently reassessing the viability of the projects under
development
with P&G due to the failure to show statistical significance of
one
of your projects.  The disclosure appears to suggest that you have
major agreements where the costs incurred on each project may be
material.  Please provide to us in disclosure type format the
disclosures required by paragraph 14 (b) of SFAS 68 related to
these
and the other collaborative projects disclosed in this note to
better
allow an investor to understand the impact that these agreements
have
on your financial statements.  Also provide in disclosure type
format
a better discussion within your liquidity section of the impact of
these agreements on your operations.

* * * *

      Please respond to this comment within 10 business days or
tell
us when you will provide us with a response.  Please furnish a
letter
that keys your response to our comment and provide the requested
information.  Detailed letters greatly facilitate our review.
Please
file your letter on EDGAR under the form type label CORRESP.

	We urge all persons who are responsible for the accuracy and
adequacy of the disclosure in the filing to be certain that the
filing includes all information required under the Securities
Exchange Act of 1934 and that they have provided all information
investors require for an informed investment decision.  Since the
company and its management are in possession of all facts relating
to
a company`s disclosure, they are responsible for the accuracy and
adequacy of the disclosures they have made.

	In connection with responding to our comments, please
provide,
in your letter, a statement from the company acknowledging that:

* the company is responsible for the adequacy and accuracy of the
disclosure in the filing;
* staff comments or changes to disclosure in response to staff
comments do not foreclose the Commission from taking any action
with
respect to the filing; and
* the company may not assert staff comments as a defense in any
proceeding initiated by the Commission or any person under the
federal securities laws of the United States.

      In addition, please be advised that the Division of
Enforcement
has access to all information you provide to the staff of the
Division of Corporation Finance in our review of your filing or in
response to our comments on your filing.

      You may contact Tabatha Akins, Staff Accountant, at (202)
551-
3658 or Jim Atkinson, Accounting Branch Chief, at (202) 551-3674
if
you have questions regarding comments on the financial statements
and
related matters.  Please contact me at (202) 551-3679 with any
other
questions.


Sincerely,



Jim B. Rosenberg
Senior Assistant Chief
Accountant
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Mr. Vikas Sinha
Alexion Pharmaceuticals, Inc.
February 7, 2006
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