Exhibit 99.1

WHITE MOUNTAINS REPORTS ADJUSTED BOOK VALUE PER SHARE

OF $565, UP 4.5% IN THE QUARTER

HAMILTON, Bermuda (April 27, 2012) — White Mountains Insurance Group, Ltd. reported an adjusted book value per share of $565 at March 31, 2012, an increase of 4.5% for the first quarter of 2012, including dividends.

Ray Barrette, Chairman and CEO, commented, “We had a good quarter with good underwriting results, solid investment returns and a lift from share buybacks.  Our investment portfolio returned 2.0% as our fixed income portfolio outperformed its benchmark but our equity portfolio lagged the S&P.  OneBeacon grew book value by 5%.  OneBeacon’s combined ratio was 94%, as a strong 89% Specialty combined ratio was offset by reserve increases in run-off.  Sirius Group’s combined ratio was 84%, an excellent result in a light catastrophe quarter.  We repurchased approximately 974,000 of our common shares at good prices, adding about $7 to adjusted book value per share in the quarter.”

Adjusted comprehensive income was $117 million in the first quarter of 2012 compared to adjusted comprehensive income of $34 million in the first quarter of last year, while net income was $94 million in the first quarter of 2012 compared to a net loss of $28 million in the first quarter of last year.

OneBeacon

OneBeacon’s book value per share increased 5.0% for the first quarter, including dividends.  The increase was driven by underwriting and investments results and a $14 million increase from the sale of OneBeacon Holdings (Luxembourg) S.à r.l. to a subsidiary of White Mountains, the effects of which are eliminated in White Mountains’ consolidation of OneBeacon.

The GAAP combined ratio was 94% for the first quarter of 2012 compared to 93% for the first quarter of last year.  The Specialty GAAP combined ratio was 89% for the first quarter of 2012 compared to 94% for the first quarter of last year, reflecting improved current accident year results. The first quarter of 2012 included 4 points of unfavorable loss reserve development compared to 2 points of favorable loss reserve development in the first quarter of last year.  The loss reserve development in the current period was driven by 5 points of unfavorable development in the run-off business, including the impact of an adverse court ruling in Mississippi regarding a disputed assessment from an involuntary pool for hurricane Katrina claims in 2005.

Mike Miller, CEO of OneBeacon, said, “We had a strong start to the year, reflecting excellent Specialty underwriting results and solid investment returns, partially offset by increased reserves in run-off.  Our Specialty written premiums grew by 17% to $303 million and we were pleased to see continued pricing momentum, along with favorable retention and new business trends.  During the quarter we launched our newest Specialty business—the OneBeacon Program Group—and we closed on the previously announced sale of AutoOne.  We are optimistic about improving market conditions and will continue to look for new specialty opportunities.”

Sirius Group

Sirius Group’s GAAP combined ratio was 84% for the first quarter of 2012 compared to 132% for the first quarter of last year.  The decrease was primarily due to lower catastrophe losses.  Catastrophe losses had minimal impact on the combined ratio for the first quarter of 2012 compared to 56 points for the first quarter of last year, which included $80 million related to the Japan earthquake and tsunami and $42 million related to a New Zealand earthquake.  Favorable loss reserve development for the first quarter of 2012 was 3 points compared to 5 points in the first quarter of last year.

Allan Waters, CEO of Sirius Group, said, “Our first quarter financial results benefited from the absence of significant catastrophe activity and from strong investment markets.  Last year’s catastrophe loss activity and model changes are firming property (re)insurance markets globally, and we are responding to our clients’ needs by growing our property book moderately.”

Gross premiums increased 5% for the first quarter of 2012 over the first quarter of last year, while net written premiums were up 4%.  The increases in premiums were driven primarily by the property and accident and health lines of business.

Other Operations

White Mountains’ Other Operations segment reported pre-tax income of $14 million in the first quarter of 2012, compared to a pre-tax loss of $15 million in the first quarter of last year.  This improvement was driven by increased net investment income and investment gains due to a higher invested asset base from the Esurance Sale proceeds, higher mark-to-market gains on the Symetra warrants and slightly lower losses from WM Life Re, somewhat offset by higher incentive compensation expenses that were driven by the increase in White Mountains’ share price during the quarter.  The value of White Mountains’ investment in Symetra warrants increased $11 million in the first quarter of 2012 compared to a decrease of $1 million in the first quarter of last year.  WM Life Re reported a pre-tax loss of $7 million in the first quarter of 2012 and a pre-tax loss of $9 million in the first quarter of last year.

White Mountains recorded a GAAP other-than-temporary impairment write-down on its investment in Symetra common shares during the fourth quarter of 2011.  As a result, White Mountains carried its investment in Symetra common shares at $15 per share at December 31, 2011, the estimate of its GAAP fair value.   During the first quarter of 2012, White Mountains’ recorded $10 million in equity in earnings from its investment in Symetra’s common shares, which increased the value of the investment in Symetra’s common shares used in the calculation of White Mountains’ adjusted book value per share to $15.56 per Symetra common share at March 31, 2012, compared to Symetra’s quoted stock price of $11.53 and Symetra’s book value per common share excluding unrealized gains and losses from its fixed maturity investment portfolio of $18.09.

Investment Activities

The GAAP total return on invested assets for the first quarter of 2012 was 2.0%, which included 0.4% of currency gains, compared to 2.0% for the first quarter of last year, which included 0.8% of currency gains.

Manning Rountree, President of White Mountains Advisors, said, “Investments are off to a good start in 2012, with a total return of 2.0% for the quarter.  Our fixed income portfolio returned 1.5%, including 0.5% related to currency movements, as we outperformed the benchmark in a quarter of rising rates and dollar weakening.  Our value-oriented equity portfolio produced a good absolute return of 4.8% but fell well short of the S&P 500 return of 12.6%.  Fixed income duration increased in the quarter from 2.2 to 2.8.  This reflects $484 million of short term investments used to fund share repurchases during the quarter.  It also reflects a sector rotation away from short tenor RMBS and toward medium tenor corporate bonds and CMBS, a move partly intended to reduce convexity risk in the portfolio.  Equity exposure is relatively flat, at 17% of the portfolio, a number we expect to increase opportunistically.”

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Capital Management

White Mountains estimates that its undeployed capital decreased from over $2 billion at December 31, 2011 to about $1.8 billion at March 31, 2012.  White Mountains repurchased and retired 974,458 of its common shares for $484 million during the first quarter of 2012 at an average price per share of $497, or approximately 88% of White Mountains’ March 31, 2012 adjusted book value per share.

David Foy, Executive Vice President and CFO, said, “We were pleased with our share repurchase activity in the quarter.  Even though the stock rose considerably, we were able to repurchase nearly a million shares at less than 90 percent of adjusted book value per share.  We continue to have tremendous financial flexibility with about $1.8 billion of undeployed capital, low financial leverage and a highly liquid investment portfolio.  This leaves us in a great position to capitalize on transactions that we find attractive.”

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 website located at www.whitemountains.com. White Mountains expects to file its Form 10-Q with the Securities and Exchange Commission on or before May 10, 2012 and urges shareholders to refer to that document for more complete information concerning its financial results.

Regulation G

This earnings release includes two non-GAAP financial measures that have been reconciled to their most comparable GAAP financial measures.  White Mountains believes these measures to be more relevant than comparable GAAP measures in evaluating White Mountains’ financial performance.

Adjusted book value per share is a non-GAAP financial measure which is derived by expanding the calculation of GAAP book value per share to exclude equity in net unrealized gains (losses) from Symetra’s fixed maturity portfolio, net of applicable taxes, from book value.  In addition, the number of common shares outstanding used in the calculation of adjusted book value per share are adjusted to exclude unearned restricted common shares, the compensation cost of which, at the date of calculation, has yet to be amortized. The reconciliation of adjusted book value per share to GAAP book value per share is included on page 6.

Adjusted comprehensive income (loss) is a non-GAAP financial measure that excludes the change in equity in net unrealized gains (losses) from Symetra’s fixed maturity portfolio, net of applicable taxes, from comprehensive income (loss) attributable to White Mountains’ common shareholders.  The reconciliation of adjusted comprehensive income (loss) to GAAP comprehensive income (loss) attributable to White Mountains’ common shareholders is included on page 7.

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

This earnings 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’:

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:

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 publicly update any such forward-looking statements, whether as a result of new information, future events or otherwise.

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WHITE MOUNTAINS INSURANCE GROUP, LTD.

CONDENSED CONSOLIDATED BALANCE SHEETS

(millions, except share amounts)

(Unaudited)

5

WHITE MOUNTAINS INSURANCE GROUP, LTD.

BOOK VALUE AND ADJUSTED BOOK VALUE PER SHARE

(Unaudited)

(1) Excludes out of-the-money stock options.

6

WHITE MOUNTAINS INSURANCE GROUP, LTD.

CONDENSED CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME

(millions, except per share amounts)

(Unaudited)

7

WHITE MOUNTAINS INSURANCE GROUP, LTD.

QTD SEGMENT STATEMENTS OF PRE-TAX INCOME (LOSS)

(in millions)

(Unaudited)

For the Three Months Ended March 31, 2012

(1) On December 31, 2011, Tuckerman Fund I was dissolved and all of the net assets of the fund, which consisted of the common stock of Hamer and Bri-Mar, two small manufacturing companies, were distributed.

For the Three Months Ended March 31, 2011

8

WHITE MOUNTAINS INSURANCE GROUP, LTD.

SUMMARY OF RATIOS AND PREMIUMS

(Dollars in millions)

(Unaudited)

(1) Total includes OneBeacon’s Other Insurance Operations, which consist primarily of business in run-off.

(end)

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