A study has found most sexually active U.S. teenagers do not use condoms, placing themselves at a higher risk of developing sexually transmitted diseases.

The study, done by the Washington non-profit research center Child Trends and reported by USA Today, used federal data collected in 2002 on unmarried teens ages 15-19.

The findings showed 47 percent of boys in the survey who had intercourse in the previous year said they always use a condom. Among girls it was 28 percent. The report is to be released Monday.

"Condom use declines a little with age and more serious relationships are less likely to use condoms," the report's co-author Jennifer Manlove was quoted as saying. "At first sexual intercourse, folks are more likely to use condoms for pregnancy and disease prevention but as they are more sexually experienced, they are more likely to switch to other methods of birth control."

One analyst told the newspaper the gender differences in condom use could be related to sex within a relationship rather than casual sex.

"My guess is you have teenage guys having more one-night stands, and guys are very likely to use a condom during a one-night stand, where with their girlfriends it's not as likely," she said.

Copyright 2006 by United Press International