For Shareholders

HAMILTON, Bermuda, Sept. 30 /PRNewswire-FirstCall/ -- White Mountains Insurance Group, Ltd. (NYSE: WTM) estimates that its total net losses resulting from the impact of hurricane Rita on its insurance and reinsurance operations will be less than $25 million pretax. When added to the range of estimates for hurricane Katrina, which was disclosed on September 9, the company expects the total net losses for the two storms will be $175 million to $325 million pretax, or about $115 million to $215 million after tax. The impact on profit commissions payable to White Mountains by Olympus Re or on the value of the company's investment in Montpelier Re are not included in these estimates.

ADDITIONAL INFORMATION

White Mountains is a Bermuda-domiciled financial services holding company traded on the New York Stock Exchange and the Bermuda Stock Exchange under the symbol WTM. Additional financial information and other items of interest are available at the Company's web site located at http://www.whitemountains.com.

Safe Harbor Statement under the Private Securities Litigation Reform Act of 1995

The press release may contain "forward-looking statements" within the meaning of Section 27A of the Securities Act of 1933 and Section 21E of the Securities Exchange Act of 1934. All statements, other than statements of historical facts, included or referenced in this release which address activities, events or developments which we expect or anticipate will or may occur in the future are forward-looking statements. The words "will," "believe," "intend," "expect," "anticipate," "project," "estimate," "predict" and similar expressions are also intended to identify forward-looking statements. These forward-looking statements include, among others, statements with respect to White Mountains':

     -- growth in book value per share or return on equity;
     -- business strategy;
     -- financial and operating targets or plans;
     -- incurred losses and the adequacy of its loss and loss adjustment
        expense reserves and related reinsurance;
     -- projections of revenues, income (or loss), earnings (or loss) per
        share, dividends, market share or other financial forecasts;
     -- expansion and growth of our business and operations; and
        future capital expenditures.

These statements are based on certain assumptions and analyses made by White Mountains in light of its experience and perception of historical trends, current conditions and expected future developments, as well as other factors believed to be appropriate in the circumstances. However, whether actual results and developments will conform to our expectations and

predictions is subject to a number of risks and uncertainties that could cause actual results to differ materially from expectations, including:

     -- claims arising from catastrophic events, such as hurricanes,
        earthquakes, floods or terrorist attacks;
     -- the continued availability of capital and financing;
     -- general economic, market or business conditions;
     -- business opportunities (or lack thereof) that may be presented to it
        and pursued;
     -- competitive forces, including the conduct of other property and
        casualty insurers and reinsurers;
     -- changes in domestic or foreign laws or regulations, or their
        interpretation, applicable to White Mountains, its competitors or its
        clients;
     -- an economic downturn or other economic conditions adversely affecting
        its financial position;
     -- recorded loss reserves subsequently proving to have been inadequate;
     -- other factors, most of which are beyond White Mountains' control.

Consequently, all of the forward-looking statements made in this earnings release are qualified by these cautionary statements, and there can be no assurance that the actual results or developments anticipated by White Mountains will be realized or, even if substantially realized, that they will have the expected consequences to, or effects on, White Mountains or its business or operations. White Mountains assumes no obligation to update publicly any such forward-looking statements, whether as a result of new information, future events or otherwise.

CONTACT: David Foy

Tel: (203) 458-5850

SOURCE White Mountains Insurance Group, Ltd.
09/30/2005

CONTACT: David Foy for White Mountains Insurance Group, Ltd., +1-203-458-5850

Web site: http://www.whitemountains.com
(WTM)