Indianapolis (AP) - Aetna Inc. said its fourth-quarter profit fell 15 percent, as it continued to struggle with rising medical costs that hurt its performance throughout 2009.

The Hartford, Conn., health insurer said Friday medical costs, its largest expense, climbed 14 percent to $6.1 billion in the three months that ended Dec. 31. That hit was partially offset by a 9 percent increase in premiums.

Aetna has said costs rose faster than it expected when it set prices for last year, due in part to the slumping economy. But CEO Ronald A. Williams said Friday morning fixes the company put in place are starting to show results.

"To date, we have repriced a significant portion of our commercial insured business and are creating momentum toward the improvement of our long-term operating margin profile," he told analysts during a conference call.

Aetna earned $165.9 million, or 38 cents per share, in fourth quarter of 2009.

That's down from the $194.7 million, or 42 cents per share, in the same period the prior year. Excluding one-time items it said profit totaled 40 cents per share.

Revenue rose 13 percent to $8.76 billion.

Goldman Sachs analyst Matthew Borsch said in a research note the insurer reported lower-than-expected earnings, but results show "clear signs" of strengthening in core operating results. He noted Aetna's cash flow is strong, and health care earnings were higher than he expected.

Aetna said claims rose partly due to treatment for swine flu and greater use of temporary COBRA insurance coverage, which people can buy after they lose their employer-sponsored insurance. Insurers say COBRA coverage is a money loser because they generally spend more on claims than they collect in premiums for it.

The insurer took fourth-quarter charges of $65 million tied to job cuts and $50 million to strengthen reserves in its disability business.

Aetna is the third-largest publicly traded health insurer based on medical enrollment. Its products include health, dental, group life and disability coverage.

For the full year, Aetna's profit fell 8 percent to $1.28 billion from $1.38 billion. On a per-share basis, profit rose to $2.84 from $2.83 because the company now has fewer shares on the market.

Its revenue rose 12 percent to $34.76 billion from $30.95 billion.

For 2010, Aetna forecast an operating profit of $2.55 to $2.65 per share. Analysts expect $2.83 per share. The insurer has said it expects 2010 operating earnings to be "modestly lower" than last year due in part to pricing pressures and Medicare Advantage reimbursement cuts.

Williams called this year a "repositioning year" that doesn't reflect Aetna's earnings potential.

Aetna competitors UnitedHealth Group Inc., WellPoint Inc., Humana Inc. and Cigna Corp. all reported fourth-quarter profits that either met or exceeded Wall Street expectations. But each also saw sizable enrollment drops compared to 2008.

Several health insurers have been hit by enrollment losses, as companies cut jobs and trimmed the number of people covered by employer-sponsored health insurance.

Aetna ended 2009 with enrollment of 18.9 million people, a 7 percent increase over 2008. But Chief Financial Officer Joseph M. Zubretsky said the insurer lost 590,000 members last year due to employer job cuts.

Aetna shares rose 33 cents to $29.56 in morning trading while the Standard & Poor's 500 index fell slightly.

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AP Business Writer Marley Seaman contributed to this story from New York.