Arch Capital (ACGL) 10-Q 2012 Q1 Quarterly report

Exhibit 15

May 10, 2012

Securities and Exchange Commission

100 F Street, N.E.

Washington, DC 20549

Commissioners:

We are aware that our report dated May 10, 2012 on our review of interim financial information of Arch Capital Group Ltd. and its subsidiaries (the “Company”) for the three-month periods ended March 31, 2012 and March 31, 2011 and included in the Company’s quarterly report on Form 10-Q for the quarter ended March 31, 2012 is incorporated by reference in the Registration Statement on Forms S-3 (Registration No. 333-158309, Registration No. 333-34499, Registration No. 333-82612, Registration No. 333-110190, Registration No. 333-117099, Registration No. 333-135421 and Registration No. 333-180329) and in the Registration Statements on Forms S-8 (Registration No. 033-99974, Registration No. 333-86145, Registration No. 333-72182, Registration No. 333-82772, Registration No. 333-98971, Registration No. 333-124422 and Registration No. 333-142835).

Very truly yours,

/s/ PricewaterhouseCoopers LLP

PricewaterhouseCoopers LLP, 300 Madison Avenue, New York, NY 10017

T: (646) 471 3000, F: (646) 471 8320, www.pwc.com/us