To evaluate Leydig cell function in men with idiopathic oligospermic infertility and its eventual role in their infertility, plasma LH, testosterone (T), 17-hydroxyprogesterone (170HP), and estradiol levels as well as plasma T/LH and 170HP/T ratios were measured in 103 such men, subdivided into different groups according to their plasma FSH levels. The results were compared to results in normal young fertile men, the subgroup of men with idiopathic oligospermic infertility who within 12 months after consultation succeeded in impregnating their partner, infertile men with a history of undescended testes (excryptorchid men), and men with Klinefelter's syndrome. As a tentative parameter of androgen insensitivity, an androgen insensitivity index [LH (IU/L) X T (nMol/L)] was calculated. Although all men with idiopathic infertility had plasma T and LH levels within the normal range, LH levels increased and T/LH ratios decreased with increasing FSH levels, while the 170HP/T and estradiol/T ratios were independent of the FSH levels and T/LH ratios. The decreased T/LH ratios in the presence of normal T levels suggest compensated Leydig cell insufficiency, which possibly contributes to the infertility. Indeed, none of the men (n = 12) with normal FSH levels but a T/LH ratio lower than 1.50 achieved fatherhood within 12 months of follow-up, although all other hormonal parameters were within the normal range. During the same period 25 men with T/LH ratios greater than 1.50 succeeded in impregnating their partners (P less than 0.05). In infertile excryptorchid men and even more so in men with Klinefelter's syndrome, plasma T levels and T/LH ratios were significantly decreased, the decrease being greater than in patients with idiopathic azospermia with similar FSH levels. None of the excryptorchid men with normal FSH levels but T/LH ratios below 1.50 fathered a child during the follow-up study. We suggest that the T/LH ratio is an additional useful prognostic parameter of infertility. Plasma T levels were increased in 15% of patients with idiopathic infertility, but increased plasma LH together with increased T levels (increased androgen resistance index) were found in only 1 man. An increased index, however, was found in 6 azoospermic excryptorchid men and 4 of 28 men with Klinefelter's syndrome. Taken together these data suggest that this index is not a reliable parameter of androgen resistance.