For Shareholders

HAMILTON, Bermuda, Sept. 9 /PRNewswire-FirstCall/ -- White Mountains Insurance Group, Ltd. (NYSE: WTM) estimates that its total net losses resulting from the impact of hurricane Katrina on its insurance and reinsurance operations will be in a range of $150 - $300 million pretax, or $100 - $200 million after tax. These estimates are based on industry losses in the range of $20 billion to $35 billion (inclusive of offshore energy losses) and are net of all reinstatement premiums. 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.

Ray Barrette, CEO of White Mountains, said, "The devastation created by hurricane Katrina is extraordinary and the financial consequences will take a long time to sort out. Our early estimates of losses are based mostly on exposure information and overall industry estimates of insured damages. Although this is a large loss for the company, we continue to have a strong balance sheet with a significant amount of undeployed capital."

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':

     X  growth in book value per share or return on equity;

     X  business strategy;

     X  financial and operating targets or plans;

     X  incurred losses and the adequacy of its loss and loss adjustment
        expense reserves and related reinsurance;

     X  projections of revenues, income (or loss), earnings (or loss) per
        share, dividends, market share or other financial forecasts;

     X  expansion and growth of our business and operations; and

     X  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:

     X  claims arising from catastrophic events, such as hurricanes,
        earthquakes, floods or terrorist attacks;

     X  the continued availability of capital and financing;

     X  general economic, market or business conditions;

     X  business opportunities (or lack thereof) that may be presented to it
        and pursued;

     X  competitive forces, including the conduct of other property and
        casualty insurers and reinsurers;

     X  changes in domestic or foreign laws or regulations, or their
        interpretation, applicable to White Mountains, its competitors or its
        clients;

     X  an economic downturn or other economic conditions adversely affecting
        its financial position;

     X  recorded loss reserves subsequently proving to have been inadequate;

     X  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) 453-1681

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

CONTACT: David Foy, +1-203-453-1681, for White Mountains Insurance Group, Ltd.

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