Clomiphene citrate, 50 mg/day, was administered to 105 patients with idiopathic azoospermia or oligoasthenospermia. Plasma FSH, LH, testosterone, and in a few cases estradiol were evaluated on day 0 and day 15 of therapy. Twelve other patients with no treatment were assayed in the same way. Statistical analysis demonstrated that, as a group, the infertile male population responds normally to clomiphene administration. On the other hand, three types of individual responses were recorded: complete positive response, dissociated positive response, and negative response. Sixty-nine patients were randomly selected for clomiphene therapy, 50 mg/day for 100 days; however, of 54 who completed the treatment schedule, only 11 showed elevated sperm counts. A study of correlations between hormonal and semen responses to clomiphene indicates that a complete or dissociated endocrine response is not an accurate predictor of possible semen amelioration on long-term therapy; on the other hand, sperm characteristics did not improve in those patients who did not demonstrate a positive hormonal response, mainly testosterone, on the 15th day of therapy.