Rise in Unprotected Sex by Gay Men Spurs H.I.V. Fears

Federal health officials are reporting a sharp increase in unprotected sex among gay American men over the last decade, a development that makes it harder to fight the AIDS epidemic.

The same trend has recently been documented among gay men in Canada, Britain, the Netherlands, France and Australia, heightening concerns among public health officials worldwide.

According to the Centers for Disease Control and Prevention, the number of men who told federal health investigators that they had had unprotected anal sex in the last year rose nearly 20 percent from 2005 to 2011. In the 2011 survey, unprotected sex was more than twice as common among men who said they did not know whether they were infected with H.I.V.

Being tested even once for H.I.V. is associated with men taking fewer risks, whether the test is positive or negative, health experts say. But the most recent survey found that a third of the men interviewed had not been tested in the past year.

The findings are worrying because “unprotected anal intercourse is in a league of its own as far as risk is concerned,” Dr. Thomas R. Frieden, director of the disease centers, said on Wednesday as the figures were released.

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