A prospective study was carried out in 156 couples attending an infertility clinic. To assess the predictive value of semen parameters in relation to pregnancy, we defined a group of 16 couples (group II) in whom the female became pregnant by intra-uterine insemination (IUI), and therefore in whom a female factor could be ruled out. Studies of semen parameters before and after capacitation were carried out in the first trimester of pregnancy (< 12 weeks). The same studies were done in the remaining 140 men (group III) with primary infertility and then all results were compared with a control group of 27 healthy, fertile men (group I), with normal semen parameters. Our results showed that progressive motility and straight line velocity were significantly lower in group III compared with group II: 33.4 and 45.2% respectively (P < 0.001) for progressive motility, and 25.7 and 32.8% respectively (P < 0.005) for straight line velocity. Acrosome alterations, on the other hand, were significantly more frequent in group III compared with group II: 21.4 +/- 0.7 and 5.9 +/- 1.7 respectively (P < 0.003). After capacitation, the recovery in terms of numbers of motile spermatozoa, spermatozoa with normal morphology and acrosome-reacted spermatozoa could be a predictive parameter of fertilization, because all were significantly decreased in group III compared with group II (P < 0.01).