ATLANTA -- The cost of treating cancer in the United States nearly doubled over the past two decades, but expensive cancer drugs might not be the main reason why, according to a surprising new study.

The study confounds conventional wisdom in several respects. The soaring price of new cancer treatments has received widespread attention, but the researchers conclude that rising costs were driven mainly by the growing number of cancer patients.

The study also finds cancer accounts for only 5 percent of total U.S. medical costs, and that has not changed in the last few decades.

"I will say I'm a bit surprised," said Dr. Len Lichtenfeld of the American Cancer Society, who said he would have expected the proportion of cancer costs to rise.

The researchers also found that private insurers now cover a greater share of cancer treatment costs -- about 50 percent -- while patients' out-of-pocket costs have fallen over the past two decades.

Though taken aback by some of the findings, Lichtenfeld and other experts did not dispute the study, which compared medical cost data from the late 1980s with that of the early 2000s. But they said the picture surely has changed in the past several years.

The study is being called the first to combine national cancer costs for all types of payers and see how they've changed.

It found that cancer treatment costs rose from nearly $25 billion in 1987 to more than $48 billion by the end of 2005.

The rise in costs is mainly because of an increase over 20 years in how many cancer patients there are, said the study's lead author, Florence Tangka of the U.S. Centers for Disease Control and Prevention.

The study did not offer precise estimates of how the number of people treated for cancer changed from the late 1980s to the early 2000s. But it showed dramatic increases in the number of cancer cases covered by the government's Medicare and Medicaid programs. Medicare, which covers the elderly and disabled, consistently has covered about one-third of the nation's cancer costs. Medicaid accounts for only 3 percent.

Better and more advanced treatments mean more people with cancer are remaining alive, so the spending increases represent money well spent, said Kenneth Thorpe, a health policy researcher at Emory University.

"It seems like we're buying increases in survival," Thorpe said.

The study is being published in Cancer, a medical journal of the American Cancer Society.