Myxedema in men is thought to cause infertility and impotence. Testicular function was investigated in eight consecutive men with primary hypothyroidism (autoimmune thyroiditis in five patients and amiodarone therapy in three patients). All had impotence that preceded the onset of hypothyroidism and did not improve with thyroid therapy. Gonadal function tests showed a hypergonadotropic state in five patients and hypogonadotropic hypogonadism in three patients including one with no response to luteinizing hormone-releasing hormone. Luteinizing hormone bioactivity was decreased in six patients and increased in two subjects who also had increased luteinizing hormone immunoreactivity. Serum testosterone and testosterone/estradiol-binding globulin concentrations were low in four of the patients. It is concluded that abnormalities of gonadal function are common in men with primary hypothyroidism.