Berkshire Hathaway, Buffett’s investment group, disclosed it had bought 5 million shares of the healthcare company earlier in the year. That sparked a 14% Friday rally in a stock that has lost more than half of its value over the past 12 months.

Warren Buffett's Berkshire Hathaway has acquired 5 million shares in UnitedHealth Group , a regulatory filing showed on Thursday, sending the health insurer's stock up 7% in extended trading.

Shares of UnitedHealth Group are surging before the market open Friday as Warren Buffett's Berkshire Hathaway disclosed that it recently purchased shares of the beleaguered insurer.