The parameters measured in the standard semen analysis may be insufficient for exact differentiation between fertile and infertile men. Therefore, we assume that the high rate of apoptotic sperm in ejaculate may play a role on the aetiology of unexplained infertility. Couples with unexplained infertility treated by ovulation induction and intrauterine insemination were consecutively enrolled (n=94). To determine the proapoptotic sperm rate, the ejaculate from patients was stained with annexin V. Thirteen of the 94 couples (13.8%) conceived after intrauterine insemination. The annexin V-positive sperm rate was found to be 20.0% in the whole group. In women failing to conceive, the annexin V-positive sperm rate was 20.8% compared to 15.7% in patients who achieved pregnancy. Although there is a trend towards higher preapoptotic sperm rate in couples failing to get pregnant with insemination, the difference did not reach statistical significance.