OBJECTIVE To determine a possible link between herpes simplex virus 1 (HSV) and infertility. METHOD A specifically designed polymerase chain reaction with nested primers, was developed and used to test for HSV in 153 men and 20 women attending an infertility clinic. RESULTS HSV DNA was detected in 37 (24%) out of 153 semen samples and in 11 (55%) out of 20 menstrual blood samples. However, HSV DNA (0%) was not detected in the semen of 16 males with children. A significant association between the evidence for infertility and an HSV positive test was observed in men (Fisher's exact test, p = 0.024), and a stronger effect was found in females after failed in vitro fertilization (Fisher's exact test p = 0.0086). CONCLUSION This is the first report of the detection of herpes simplex virus DNA in semen and menstrual blood. Encouraging preliminary results justify antiviral therapy in case of a positive test.