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.

The data, published in the agency’s Morbidity and Mortality Weekly Report, do not explain why unprotected sex has risen so rapidly, but a leading hypothesis, Dr. Frieden said, is that more men are “sero-sorting” — that is, those who are uninfected (“H.I.V. seronegative” on lab reports) try to sleep only with other men who are uninfected, or who hope they are, or who merely promise they are.

“The problem with sero-sorting is that it’s really easy to get it wrong,” Dr. Frieden said. “When one-third of men aren’t even tested in the last year and a tenth of those who thought they were negative were actually positive, you don’t want to risk your life on a guess.”

Other hypotheses, say Dr. Frieden and Dr. Jonathan Mermin, the disease centers’ director of H.I.V. prevention, are that many young men have never known anyone dying of AIDS and so do not fear it, or that they believe that they can easily stay on antiretroviral drugs for life.

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Two leading independent AIDS researchers agreed only partly with those explanations.

“Young guys are less worried,” said Alex Carballo-Diéguez, a researcher at the H.I.V. Center of the New York State Psychiatric Institute and Columbia University who has studied gay men’s behavior since the 1980s. “H.I.V. has become a chronic disease, and everyone knows some behaviors are bad for you, like smoking and trans fats. But in the moment of excitement, they’re going to do what they enjoy.”