To compare the efficacy and safety of nandrolone decanoate and calcium (NDC) with those of calcium alone (CAL) in men with idiopathic osteoporosis, a 12-month, randomized, prospective, controlled study, was performed in an outpatient clinic. Twenty-one men with idiopathic osteoporosis (as determined by radiological and dual energy x-ray absorptiometry findings) were randomly allocated to either 50 mg nandrolone decanoate intramuscularly (im) weekly and 1,000 mg oral calcium carbonate daily (NDC group) or to 1,000 mg oral calcium carbonate daily (CAL group). Bone densitometry (total body, left femur, and lumbar spine), serum, and urine biochemical parameters were measured at 3-month intervals. In the NDC group, bone mineral density initially increased, reached a plateau, and then decreased to near baseline levels at 12 months. Increases in lean muscle mass mirrored these changes. Free and total testosterone significantly decreased. Hemoglobin increased in all patients in this group. Patients in the CAL group exhibited no significant change in either total body or bone mineral density or biochemical parameters. Thus, nandrolone decanoate, 50 mg im weekly, transiently increases the bone mass of men with idiopathic osteoporosis in this preliminary study. Careful monitoring is necessary.