Selenium (Se) was measured in the semen of 125 men from couples consulting for infertility. A mean concentration of 71.3 +/- 29.7 ng/ml of semen was found, with a range of 7 to 230 ng/ml. More than 85% of the Se is in the seminal plasma. There was a significant positive correlation between sperm count and semen Se. Sperm motility was maximal at semen Se levels ranging between 50 and 69 ng/ml; above and below this range, motility was decreased and the incidence of asthenospermia was high. This result suggests an optimal range for semen Se. A follow-up of 4.5 to 5 years after the initial assay of Se revealed that low semen Se levels (less than or equal to 35 ng/ml) were associated with male infertility. A Se level between approximately 40 and 70 ng/ml was optimal for reproductive performance (high pregnancy rate and low abortion rate). Semen Se levels greater than or equal to 80 ng/ml were associated with a high abortion rate and signs of ovarian dysfunction in the partner (both partners usually have the same diet and environmental exposure). These results attest to the role of Se in human reproduction, a well-established fact in several animal species. The semen Se level appears to be a useful indicator of Se status versus reproductive function. Further studies are warranted concerning the general aspects of metabolism and mechanism of action of Se in infertile couples before any therapeutic modification of intake of this element can be contemplated.