1977: UnitedHealthCare Corp. becomes a legal, for-profit entity and begins buying and managing HMOs. Businessman Richard Burke had launched the company in 1974 to manage the nonprofit Physicians Health Plan of Minnesota, a forerunner of Medica.

1984: Company becomes publicly traded. United has 200 employees, $7 million in revenue, and operates 11 HMOs in 10 states.

1987: Burke resigns as CEO after doctors at Physicians Health Plan, the state's largest HMO where he also serves as chief executive, accuse him of conflict of interest. Burke remains on the board.

1989: Bill McGuire takes over as president after the company suffers a loss of $36.7 million in 1988. He cuts costs and invests in technology.

1990 to 2000: Company sees a decade of rapid growth under McGuire, who becomes chairman and CEO in 1991. United grows from a regional player with $605 million in revenue to a $21 billion firm by the end of the decade, with a growing Medicare business.

1998: Company is renamed UnitedHealth Group. A plunge in its stock price puts the skids on a planned $5.5 billion merger with Humana, which would have created the country's largest managed-care company.

October 2006: McGuire steps down as CEO and board chairman after an investigation by the Securities and Exchange Commission into stock-option backdating. The company pays $900 million to settle shareholder lawsuits. Burke, the founder, returns as chairman.

December 2006: Stephen Hemsley takes over as CEO. He had come to the company from Andersen Consulting in 1997 and became president in 1999.

2007 to 2010: United buys up smaller technology companies, ramping up health services business.

2011: Revenue exceeds $100 billion mark.

Sources: SEC documents, UnitedHealth Group, CRT Capital Group analyst Sheryl Skolnick/Columbia University.