ABSTRACT: The sleep apnea syndrome, a condition predominantly affecting males, is associated with impotence (in 42% to 48% of the patients) and low serum testosterone. We investigated the endocrine control of testicular function in six patients with sleep apnea, impotence, and low serum testosterone levels (72–280 ng/dl; normal range 300–1000). Serum free testosterone, calculated from total testosterone and testosterone-estradiol binding globulin concentrations, was low in five of the subjects. Luteinizing hormone (LH) concentration was normal and responded normally to its releasing hormone; however, the LH bioactivity/immunoreactivity ratio (BIO/RIA) was subnormal in all the tested cases (4:4). Active spermatogenesis was found in three patients.Thus, the impotence and low concentrations of total and free testosterone of the sleep apnea syndrome represent a state of hypogonadotropic hypogonadism. This type of hypogonadism appears to be secondary to decreased steroidogenic activity of circulating luteinizing hormone.