We have examined the potential of follicle-stimulating hormone (FSH) therapy for the male to improve pregnancy rates in intrauterine insemination (IUI) with husband's spermatozoa. A prospective randomized trial was performed in 148 couples undergoing IUI because of male subfertility. In the treatment group, 150 IU FSH were administered to the husbands, either i.m. or s.c., three times a week, starting 3 months before the beginning of IUI cycles and maintained until the fifth IUI cycle. In the control group no treatment was given. FSH therapy did not change semen parameters. The pregnancy rate per cycle was 13.47% in the FSH group versus 10.07% in the non-FSH group; the pregnancy rate per woman was 44.38% in the FSH group versus 37.18% in the non-FSH group. Although the pregnancy rate increase was > 30% per cycle and > 20% per woman, statistical significance was not achieved. The cumulative pregnancy rate was 59.20% in the FSH group versus 42.91% in the non-FSH group. The pregnancy rate outside the IUI cycle was 14.70% (10/68) in the FSH group versus 2.5% (2/80) in the non-FSH group, the difference being statistically significant. In conclusion, a non-significant trend towards higher pregnancy rates in IUI was observed in the FSH group.