#10 Centene
Ticker: CNC
YTD stock performance: 48%
Market cap: $1.8 billion
Fortune 500 Rank: 493

Shares of Centene, which provides managed care programs and related services to individuals under Medicaid, surged as the company continued to add more members and expand into new states, including Florida, Illinois, Mississippi and Texas.

In its last quarterly report, St. Louis-based Centene boasted 1.6 million members, up almost 8% from a year ago. Investors were also pleased that the company raised its outlook for the year.

Centene's stock has also benefited from talk that the managed-care provider could be ripe for a takeover, following a slew of acquisitions by major health insurers of smaller companies that specialize in covering Medicaid patients.

Wells Fargo analysts say the company is also likely to benefit from health reform, since it is set to substantially expand Medicaid eligibility. Cash-strapped states have already started looking into shifting recipients to managed-care providers, which have promised to keep costs low.

By Maureen Farrell and Hibah Yousuf @FortuneMagazine - Last updated December 22 2011: 11:33 AM ET
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