Linda Valleroy and colleagues did a five-year CDC study of 3,492 men in seven US cities who have sex with men, and found one in six also had sex with women.
A quarter of those men, aged 15 to 22, said they recently had unprotected sex with both men and women, Valleroy’s team told the 13th International AIDS Conference. She said nearly seven percent of the men in the study were HIV positive.
"The study confirms that young bisexual men are a ‘bridge’ for HIV transmission to women," the CDC said in a statement.
CDC researchers said they were confirming studies that show a worrying rise in risky behaviour among gay and bisexual men — who are still the main victims of HIV in the United States.
Dr Paul Denning of the CDC and colleagues said they had interviewed 1,942 gay or bisexual men living in 12 cities who had just been diagnosed with HIV.
They said 19 percent had at least one episode of unprotected anal sex — the riskiest sexual behaviour — in the year before in 1997 and 1998. That is a 50 percent rise from 1995 and 1996, when only 13 percent of men said they had unprotected anal sex.
"Gay men of all ages remain at an alarming risk," Dr David Holtgrave of the CDC told a news conference.
He said one problem was that men believed drug cocktails that suppress the virus prevented its spread. "They perceive that sex partners with low viral loads present less risk than men with high viral loads," Holtgrave said.