The sexually transmitted infection chlamydia can affect a man's fertility, researchers have found.
Men should be aware their fertility can also be affected by chlamydia
It is known that if a woman has the often symptomless infection, her own fertility can be affected.
But researchers from Sweden's Umea University suggest male infection reduced a couple's chance of having a baby by around a third.
Writing in Human Reproduction, they say this was not because the man passed the infection on to the woman.
Research recently suggested that one in 10 young men in the UK were infected with chlamydia.
White or cloudy, watery discharge from the tip of the penis
Pain or a burning sensation when passing urine
Testicular pain and/or swelling
The researchers tested 244 infertile couples attending a clinic for fertility treatment.
They tested both men and women for chlamydia antibodies. If these were found in blood samples, it would show a current or past chlamydia infection.
For those couples where one partner was positive they also tested for chlamydia DNA.
The couples were monitored for progress for an average of 37 months.
Researchers also followed a group of women in the same age range who had conceived naturally.
They found antibodies in nearly a quarter of the infertile women, a fifth of the infertile men but only 15% of those who had conceived naturally.
Seven per cent of infertile men and 7% of infertile women who had the antibodies also carried chlamydia DNA in their urine, suggesting active infection.
Antibodies in the women were related to tubal factor infertility (TFI) which affects the fallopian tubes - confirming previous findings that chlamydia is linked to TFI.
But antibodies in men were not associated with TFI in their partners.
Professor Jan Olofsson said: "Importantly, as well as the expected finding of antibodies among the female partners we found that antibodies in the male partner was significantly inversely correlated to the overall pregnancy rate. "
"Our findings show that it is not only women that need to be concerned about contracting chlamydia.
"Men need to be aware that this is potentially serious for them as well."
He added: "As there was no connection between antibodies in the men and TFI in their partners there may be alternative or additional mechanisms involved that are reducing fertility."
Professor Olofsson added it was not clear what effect chlamydia had on men, but added: "It is possible that decreased sperm motility or concurrent or undetected infection may play a role.
When chlamydia-affected couples did conceive, either naturally or through IVF, neither active infection or chlamydia antibodies affected the pregnancy.
An unusual vaginal discharge
Pain when passing urine
Bleeding between periods
Pain during sex or bleeding after sex
Low abdominal pain
The researchers said the study showed both men and women attending fertility clinics should be screened for chlamydia.
Dr Allan Pacey, a specialist in male fertility at the University of Sheffield, told BBC News Online the Swedish findings were significant.
"The presence of antibodies in the male partner was negatively related to pregnancy outcome."
He said more research would have to be carried out to find out why this was the case.
But Dr Pacey, a member of the British Fertility Society, added: "It suggests that chlamydia has quite a subtle effect on sperm that we're unable to detect."
He said the national chlamydia screening programme, which focuses on women, should be extended so men were also tested.
"Men should also take more responsibility for their sexual health."
Dr Sreebala Sripada of the University of Aberdeen said: "This study certainly calls for further research to address this basic question as to whether chlamydia infection in the male affects fertility and the need is urgent due to the escalating trends in the prevalence of genital chlamydia."