Increased melatonin secretion observed in male patients with congenital isolated hypogonadotropic hypogonadism and its normalization during testosterone treatment had suggested that melatonin and the reproductive hormones are inter-related. Since these patients have a congenital form of hypogonadism, it is likely that hypermelatoninemia is the consequence of hypogonadism. To further study the relations between the pineal and the reproductive axis in humans, we evaluated melatonin secretion in two men (aged 35 and 50 yrs.) with acquired adult-onset hypogonadotropic hypogonadism. The diagnosis was based on the findings of normal testicular volume, azoospermia, low serum testosterone, normal LH and FSH levels, but apulsatile LH secretion, and intact anterior pituitary hormones secretion, normal findings on skull radiographic imaging, prior sexual maturation and paternity. Melatonin secretion was assessed as urinary 24 h 6-sulphatoxymelatonin excretion (aMT6s) prior to and during the administration of 250 mg testosterone enanthate per month for 4 months. Pretreatment melatonin production was markedly increased in both patients: 427-915 ng/kg/24 h vs. 204+/-81 [mean+/-SD] in 16 age-matched male controls. During testosterone treatment, aMT6s levels were normalized in one patient (range: 81-287 ng/kg/24 h) and remained elevated in the other patient (range: 830-1280 ng/kg/24 h). These data indicate that male patients with acquired GnRH deficiency have increased melatonin secretion. Melatonin hypersecretion in these patients may reflect a functional association.