In many patients with involutional osteoporosis, anabolic steroids may produce a rapid subjective improvement and pronounced reduction of complaints. In animal experiments it has been demonstrated that anabolic steroids can also have objective effect on bone tissue. Twenty postmenopausal osteoporotic patients have been randomly assigned to two different treatments: 10 patients were treated with 50 mg i.m. of nandrolone decanoate every 3 weeks for 12 months; 10 patients were treated with placebo. Both groups received an oral calcium supplement (1 g/day). Bone mineral content (BMC) was measured by dual photon absorptiometry before and after 1, 3, 6 and 12 months. Plasma alkaline phosphatase (A.Ph.) and urinary hydroxyproline/creatinine ratio (HOP) were measured at the same times. Intestinal calcium absorption was measured by the 47Ca oral test before and after treatment. In 4 patients of both groups a transiliac bone biopsy was performed before and after treatment. After 1 year there was a significant increase in the BMC of the lumbar spine in the group receiving calcium plus nandrolone decanoate. A progressive but not significant increase of A.Ph. was observed in the group treated with nandrolone decanoate. Radiocalcium absorption significantly increased in nandrolone treated patients. The histomorphometric study of bone demonstrated a significant increases in trabecular bone volume and in active osteoid surfaces in patients treated with nandrolone decanoate. Because the plasma A.Ph. tendes to increase with no change in bone resorption (as measured by urinary HOP) and the active osteoid surfaces significantly augment, we conclude that nandrolone therapy increases the bone formation rate.(ABSTRACT TRUNCATED AT 250 WORDS)