Chlamydia can affect fertility in both men and women
One in five men in their early 20s in England has the sexually transmitted infection chlamydia, figures suggest.
Men aged 20 to 24 were twice as likely to test positive as under-19s, the Health Protection Agency study found.
Some 16,413 under-25s were tested from April 2003 to March 2004 in the first year of a national screening programme.
Overall, one in eight men and one in 10 women tested positive for chlamydia - often known as the silent infection because it can have no symptoms.
The researchers found teenage women were 43% more likely to test positive than women in their early 20s, the journal Sexually Transmitted Infections reported.
Screening is offered at traditional venues such as family planning clinics and GP surgeries as well as colleges and universities.
The programme has been introduced after the number of chlamydia cases almost tripled between 1995 and 2003.
The infection can cause pelvic inflammatory disease and ectopic pregnancy in women and infertility in both men and women.
Scott LaMontagne, from the HPA's Communicable Disease Surveillance Centre, said the findings for men were higher than expected.
"We may find as we test more men this figure drops but nonetheless it is clear there are a large number of cases of this infection in men and women.
"Hopefully this will act as a wake-up call. People need to get themselves tested."
He said one of the problems with the disease was that 70% of women and 50% of men did not show any noticeable symptoms.
One in 10 women (10.1%) tested positive for chlamydia
Teenage women were 43% more likely than women in early 20s to test positive
One in eight men (13.3%) tested positive
19.8% of men aged 20 to 24 tested positive, compared to 10% of teenagers
"Many people are unaware they have it."
A Department of Health spokeswoman said the findings proved the screening programme was the right approach.
"This data will also help us to improve understanding of the distribution and determinants of chlamydia infection, as well as providing lessons which will enhance the subsequent phases of the roll-out."
And a Family Planning Association spokeswoman said the overall figures did not come as a surprise.
"Chlamydia is the most commonly diagnosed STI in the UK, so it is important that men and women are aware of the risks of unprotected sex.
"We urge the government to step up investment and develop more sexual health campaigns to raise awareness, especially within the high risk groups identified by the programme."
The researchers also found that having a new sexual partner or having slept with more than two people in the last year was common among the young people being tested.
In women, almost half said they had had a new sexual partner in the preceding three months and/or two or more sexual partners in a year.
For men the figures were even higher - 56% reported a new sexual partner in the last three months and 60% reported two or more sexual partners in the past year.
The researchers said there was still some way to go before the screening programme led to a reduction in infections as seen in Sweden and the US where young people are already routinely tested.
The under-25s were tested in 302 centres across 10 areas.
The remaining 16 programme areas are expected to be covered by 2008.