TORONTO - Teens who use condoms the very first time they have sex are half as likely to test positive for sexually transmitted diseases several years later, a U.S. study reported Thursday.

The survey involved more than 4,000 sexually active adolescents who were questioned between 1994 and 2002.

In addition to having lower rates of chlamydia and gonorrhea, the individuals who were followed were 36 per cent more likely to say they had used a condom in their most recent sexual encounter seven years later.

Sixty-two per cent of the respondents reported having used a condom when they had intercourse for the first time.

"Adolescents who use condoms at their sexual debut do not report more sexual partners, are more likely to engage in subsequent protective behaviours, and experience fewer sexually transmitted infections than do adolescents who do not use condoms at their sexual debut,'' concluded the study by researchers at the University of Washington, Seattle.

The findings were published in the American Journal of Public Health.

Dr. Tom Wong, director of the Community Acquired Infections Division at the Public Health Agency of Canada, said studies in this country have had similar results.

He stressed the importance of educating young people about the possible health consequences of unprotected sexual intercourse.

"It's actually a big problem, and certainly a challenge in the United States as well as in Canada,'' he said from Ottawa.

"Take chlamydia, for example. Every year, right now we have over 60,000 cases reported and those are just reported. And so it's a tip of the iceberg, because there are cases not diagnosed and not reported.''

"Of those cases, the majority are happening in young people under the age of 25.''

Problems that can arise include infertility, ectopic pregnancies, chronic pain and pelvic inflammatory diseases.

Wong noted that gonorrhea is becoming resistant to all but one class of antibiotics.

And there have been a number of different syphilis outbreaks in Canada.

"One of the things that's happening, in addition to affecting the individual, if one is pregnant and has syphilis, one can pass it on to the newborn,'' he said.

"And most recently we are seeing an increase in the incidence in the number of cases of babies born with what we call congenital syphilis. And it used to be very rare until just recently.''

The overriding message, he suggested, is that young people should not take risks and should make good choices.

"Sometimes young people think they're invincible and so that's not perhaps the best way to approach sexual relationships,'' Wong said.

"Sexual relationships -- it's not just about protection or no protection. It's also about emotional readiness, and so on and so forth. I think that it would be good for young people to be discussing with their counsellors and their health care providers about ways that they could protect themselves if they choose to be sexually active.''