Buffett owned UnitedHealth shares before selling his stake in 2010, though his Berkshire Hathaway recently acquired shares.

The blue-chip Dow opened at a record high on Friday, as UnitedHealth's shares jumped after Berkshire Hathaway raised its stake in the health insurer, while expectations of an interest rate cut in September also lifted the mood.

Rising costs in the broader healthcare sector and an about 46% slump in heavyweight UnitedHealth's shares this year have left the Dow lagging its Wall Street peers on the road to record highs. The price-weighted index last scaled an all-time high on December 4.