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DALLAS --

Flying against the headwinds of a recession and volatile fuel prices, Southwest Airlines Co. made money in the fourth quarter and extended its string of annual profits to 37 years.

Southwest said Thursday it earned $116 million in the quarter, which was enough to lift the company to a full-year profit of $99 million after it lost money during the first nine months of 2009.

Traffic on the nation's largest discount carrier has been rising in recent months while it's been falling on many competitors.

But many of Southwest's customers are leisure travelers responding to the airline's constant fare sales. Chairman and CEO Gary C. Kelly said a "dramatic" drop in higher-priced business travel during the recession has made this "perhaps, the most difficult revenue environment the airline industry has ever faced."

Kelly said 2010 will be challenging too, with a tepid economy so far and higher fuel costs.

Traffic rose at Southwest each month of the fourth quarter, with paying passengers flying 5.3 percent more miles than in the same period of 2008. There were fewer flights and fewer empty seats.

Southwest believes it is taking customers from other airlines by letting passengers check two bags for free, but it has boosted ticket revenue by charging fees for services such as flying unaccompanied minors and pets.