Stephen Hemsley ran UnitedHealth Group for more than a decade. It's a different company after four years with Andrew Witty in charge.

Former CEO and current chairman Stephen Hemsley is stepping back into the top job following the abrupt resignation of Andrew Witty, UnitedHealth said.

UnitedHealth Group Inc UNH announced Tuesday morning that its CEO, Andrew Witty, will resign for personal reasons.. Stephen Hemsley, who previously held the CEO title from 2006 to 2017, will succeed him. Hemsley will also remain chairman of UnitedHealth's board of directors, and Witty will become senior adviser to the CEO.

Even UnitedHealth will struggle to overcome parasitic medical costs that Warren Buffett once called a tapeworm eating away at U.S. economic competitiveness. The $300 billion healthcare conglomerate reinstalled Chairman Stephen Hemsley as CEO and yanked its financial guidance.

UnitedHealth CEO Andrew Witty is stepping down for personal reasons and the nation’s largest health insurer suspended its full-year financial outlook due to higher-than-expected medical costs. Hemsley was UnitedHealth Group CEO from 2006 to 2017. He will remain chairman of the company’s board. Witty will serve as a senior adviser to Hemsley.

UnitedHealth Group shares rebounded modestly Wednesday following yesterday's crash, with analysts from UBS, Oppenheimer, and Morgan Stanley leaving their ratings unchanged after the health insurer pulled its outlook and announced its CEO had stepped down.

UnitedHealth Group, the largest health insurer in the country, is turning back the clock in a moment of crisis. On Tuesday, the company announced that chairman Stephen Hemsley is returning as CEO, stepping back into the role he left in 2017.

UnitedHealth Group surprises investors with what its CEO said was an "unusual and unacceptable" quarterly earnings miss, and lowers its outlook for the full year due to higher-than-expected medical costs, sparking a more than 20% selloff in shares that reverberated across the sector.