The effects of nandrolone decanoate (ND; 50 mg IM every three weeks) on calcium metabolism and forearm bone density were studied in a randomized trial in 35 women receiving long-term therapy with corticosteroids (CST) for rheumatic disease. The 17 patients who served as controls were on CST therapy for less years and their bone density was higher. Thus a second control group, pair-matched with the active treatment group for age, duration of CST therapy and bone density, was selected retrospectively. At the end of the 18 months' treatment course with ND, forearm bone density was increased by 5.1% (P less than 0.01) but fell by 11.3% (P less than 0.01) and 6.7% respectively in the first and second control group. The patients on ND differed significantly from both control groups in the changes at 6, 12 and 18 months (P less than 0.01). Urinary excretion of hydroxyproline fell significantly in patients receiving ND, whereas the biochemical indices of bone formation did not change (alkaline phosphatase) or increased (osteocalcin; P less than 0.01). In conclusion, nandrolone decanoate therapy may be used in the prevention of CST-induced osteoporosis. It also seems to exert mild inhibition of bone resorption without affecting or even stimulating bone formation.