About one in five ninth-graders in the U.S. say they've had oral sex, a finding that adults should keep in mind when counselling teens about sex, researchers say.

The study, which appears in the April issue of the journal Pediatrics, was designed to gauge teen perceptions of oral sex versus vaginal sex.

To find out, researchers surveyed 580 ethnically diverse ninth-graders in California with an average age of 15½. Girls made up 58 per cent of those surveyed.

Caro and Oliver, both 17, were featured in a documentary on teen relationships.

Of the respondents, 20 per cent said they had engaged in oral sex, compared to 14 per cent who said they had had sexual intercourse.

One-third also said they intended to have oral sex within the next six months.

"These findings suggest that adults should discuss more than one type of sexual practice when they counsel teens," said Bonnie Halpern-Felsher, a professor of adolescent medicine at the University of California, San Francisco, who led the research.

Girls and boys reported similar experiences and opinions about oral sex, said Halpern-Felsher.

Scientists have largely learned of the risks of oral sex based on case reports and studies of HIV transmission among gay men.

Little is known about the risks for teens, although oral sex has the potential to spread herpes, hepatitis, gonorrhea, chlamydia, syphilis and HIV.

Although the risk of a sexually transmitted disease is less from oral sex than from intercourse, earlier studies suggest teenagers likely underestimate the risk, the study said.

"Given the suggestion that adolescents do not view oral sex as sex, and see oral sex as a way of preserving their virginity while still gaining intimacy and sexual pleasure, they are likely to interpret sexual health messages as referring to vaginal sex," the pediatrician wrote.

"Adolescents also believed that oral sex is more acceptable than vaginal sex for adolescents their own age in both dating and non-dating situations, oral sex is less of a threat to their values and beliefs, and more of their peers will have oral sex than vaginal sex in the near future."

The 2002 National Survey of Family Growth asked about oral sex and is expected to offer more data on the sexual practices of U.S. teens.