To evaluate the effects of treatment with FSH on seminal parameters and spontaneous pregnancy in male infertility. Design: Prospective, controlled, randomized clinical study. Setting: Infertility center at a university hospital. Patient(s): One hundred twelve men affected by idiopathic oligozoospermia. Intervention(s): Patients were randomized into two groups: 62 subjects were treated with 100 IU of recombinant human FSH on alternate days for 3 months, and 50 patients did not receive any treatment. Semen analysis was performed in all subjects at the end of this period of treatment and after the following 3 months. Subjects who had not reached spontaneous pregnancy underwent assisted reproductive techniques. Main Outcome Measure(s): Seminal parameters, testicular cytologic analysis, FSH, LH, T, and inhibin B concentrations. Result(s): The treatment group considered as a whole did not show modifications in sperm parameters. However, a subgroup of these (30, 48.4% ) had a significant increase of sperm count (responder group). In the period including 3 months after the withdrawal of FSH therapy, we observed a significantly higher spontaneous pregnancy rate in the responder group (5 of 30 [16.7% ]) with respect to nonresponder and nontreated groups (1 of 32 [3.1% ] and 2 of 50 [4.0% ], respectively). Furthermore, the improvement of seminal parameters in the responder group allowed these patients to undergo less frequent IVF- ET/intracytoplasmic sperm injection. Conclusion(s): Results from this controlled, randomized clinical trial show that FSH therapy does not improve sperm concentration or pregnancy rate when infertile male patients are chosen solely by the clinical criteria of idiopathic oligospermia and normal FSH concentration. Subgroup analysis, however, does indicate that patients without maturation arrest in addition to the clinical scenario do benefit from medical therapy. To evaluate the effects of treatment with FSH on seminal parameters and spontaneous pregnancy in male infertility. Design: Prospective, controlled, randomized clinical study. Setting: Infertility center at a university hospital. Patient (s): One hundred twelve men affected by idiopathic oligozoospermia Intervention (s): Patients were randomized into two groups: 62 subjects were treated with 100 IU of recombinant human FSH on alternate days for 3 months, and 50 patients did not receive any treatment. Semen analysis was performed in all subjects at the end Subjects who had not reached spontaneous pregnancy underwent assisted reproductive techniques. Main Outcome Measure (s): Seminal parameters, testicular cytologic analysis, FSH, LH, T, and inhibin B concentrations. (s): The treatment group considered as a whole did not show modifications in sperm parameters. However, a subgroup of these (30, 48.4%) had a significant incr ease of sperm count (responder group). In the period including 3 months after the withdrawal of FSH therapy, we observed a significantly higher spontaneous pregnancy rate in the responder group (5 of 30 [16.7%]) with respect to nonresponder and nontreated groups (1 of 32 [3.1%] and 2 of 50 [4.0%], respectively). Furthermore, the improvement of seminal parameters in the responder group allowed these patients to undergo no more frequent IVF- ET / intracytoplasmic sperm injection. Conclusion (s) : Results from this controlled, randomized clinical trial show that FSH therapy does not improve sperm concentration or pregnancy rate when infertile male patients are chosen solely by the clinical criteria of idiopathic oligospermia and normal FSH concentration. Subgroup analysis, however, does indicates that patients without maturation arrest in addition to the clinical scenario do benefit from medical therapy.