Background: HCA Holdings ( HCA), the parent company of the Hospital Corp. of America, is a fairly new addition to the top 10 list. The health care services operator underwent an IPO in March 2011, raising $4.4 billion, after being taken private in 2006. Headquartered in Nashville, HCA operates and owns 163 hospitals and 109 freestanding surgery centers as well as diagnostic and imaging centers, radiation and oncology therapy centers, rehabilitation and physical therapy centers and other facilities, according to its profile. HCA describes itself as the nation's leading provider of healthcare services. Approximately 4% to 5% of all inpatient care delivered in the country today is provided by HCA facilities, the company says. Stock performance: Shares of HCA have fallen more than 17% since its debut as a public company last year. The stock was up 3.98% Monday to $25.61.