JANA disclosed ownership of 3.8 million shares of health insurer Aetna Inc (NYSE:AET) at the end of March. At a market capitalization of $21 billion, the stock trades at 13 times trailing earnings; health insurers are generally a bit cheap in the current market as investors worry about future regulation of the industry if health care costs continue to rise. In the first quarter of the year Aetna Inc (NYSE:AET)'s earnings fell slightly despite 7% revenue growth compared to a year ago. Greenlight Capital, managed by David Einhorn, reported a position of 6.5 million shares of Aetna Inc (NYSE:AET) in its own 13F (find Einhorn's favorite stocks).