STUDY OBJECTIVE: To determine the effects of alteration of gonadal steroids on bone mass in men with hyperprolactinemic hypogonadism. DESIGN: Prospective survey of patients for a median period of 33 months. SETTING: Neuroendocrine clinical center of a referral-based university medical center. INTERVENTIONS: Reversal of hyperprolactinemia by bromocriptine therapy, transsphenoidal surgery, or radiation therapy, alone or in combination. PATIENTS: Consecutive sample of 20 male patients with prolactin-secreting pituitary tumors. MEASUREMENTS AND MAIN RESULTS: In patients who had a reversal of hyperprolactinemia and a restoration of gonadal function, a significant increase in bone density of the radial shaft was noted (mean +/- SE, 0.77 +/- 0.03 to 0.84 +/- 0.03 g/cm2; P less than or equal to 0.05). These patients also had a minimal change in the density of vertebral bone (109 +/- 9 to 115 +/- 10 mg of potassium phosphate, dibasic/cm3). Patients who remained hypogonadal despite the reversal of hyperprolactinemia had no change in radial (0.76 +/- 0.03 to 0.76 +/- 0.04 g/cm2) or vertebral bone density (105 +/- 10 to 103 +/- 9 mg of potassium phosphate, dibasic/cm3). There was a positive correlation between the change in testosterone level and the change in radial (r = 0.67, P less than or equal to 0.003) or vertebral bone density (r = 0.70, P less than or equal to 0.003). CONCLUSIONS: The reversal of hypogonadism, independent of the prolactin concentration, is associated with an improvement in bone mass. Our findings emphasize the importance of gonadal steroids in maintaining skeletal integrity in men.