In males, testosterone is normal or decreased in hypopituitarism, Kallmann's syndrome, isolated gonadotrophin deficiency and Klinefelter's syndrome. It is normal in cryptorchidism, azoospermia and oligospermia. It is decreased in delayed puberty, primary testicular failure and anorchia. Testosterone is normal or increased in complete testicular feminization syndrome. Testosterone is increased in precocious puberty, related to idiopathic or CNS lesion, or to adrenal tumors or congenital adrenal hyperplasia. Testosterone exists in serum both free and bound to albumin and to sex hormone binding globulin. The latter is increased by estrogen and thyroid hormone and is decreased by androgen excess. In females, testosterone provides useful information in evaluating hirsutism and virilization. Many hirsute women exhibit normal testosterone levels in peripheral circulation. It is therefore highly recommended that binding index (free testosterone) be determined in cases of suspected androgen excess. Determination of testosterone and androstenedione in adrenal and vein plasma may be helpful in locating the site of excess androgen production. In males, it is useful in evaluating impotence, hypogonadism, delayed and precocious puberty. In general, there appears to be little advantage in doing urine testosterone measurements compared to (or in addition to) serum measurements and the serum test is recommended.