Three hundred sixty-nine infertile couples were followed for 2 to 5 years in a study designed to determine the clinical long-term predictive value of the zona-free hamster ova sperm penetration assay (SPA). Semen analysis (SA), SPA, and a full infertility workup were done in all cases, and only couples in whom the female had no evident cause of infertility were included in the study. During the follow-up period, 106 couples (29%) achieved a pregnancy. Sixteen percent of 131 men who had an SPA of 0%, 23% of 120 men with 1% to 19%, and 48% of 118 men who had a penetration of greater than 19% impregnated their wives 2 to 5 years after the assays. Significant difference in fertility prognosis was found between those who had an SPA greater than 19% and those with an SPA less than 20% (48% versus 20%). Sperm penetration assay greater than 19% was predictive of higher pregnancy rates in both oligospermic (41% versus 17%) and unexplained infertile couples (52% versus 24%). The specificity and positive predictive values of the SPA were higher than those of the SA (77% versus 57% and 48% versus 37%). These findings emphasize the value and importance of the SPA in determining the long-term fertility potential of men.