To assess Leydig cell function in men with idiopathic oligospermia, serum levels of testosterone and 17-hydroxyprogesterone were measured before and after the administration of human chorionic gonadotopin (hCG) to men with oligospermia (< 20 x 10(6) sperm/ml). Serum testosterone and 17-hydroxyprogesterone levels, both basally and after 2 or 72 hours of hCG stimulation of Leydig cell output, were the same in the oligospermic and control groups, although basal serum testosterone levels correlated directly with sperm density (r = 0.34; P < 0.05). The basal serum level of luteinizing hormone (LH) was slightly but significantly higher in the oligospermic group than in the control group (12.0 +/- 1.0 [SE] mIU/ml versus 9.5 +/- 0.5; P < 0.05), as was the LH:testosterone ratio (2.45 +/- 0.20 versus 1.74 +/- 0.20; P < 0.02). The combination of slightly elevated serum LH levels with unchanged basal serum levels of gonadal steroids suggests that men with idiopathic oligospermia have a mild, compensated Leydig cell defect. The mildness of this defect is reflected by the ability of maximal gonadotropic stimulation to increase androgen output.