Long-acting medroxyprogesterone acetate (MPA) effect on some important parameters of calcium metabolism in patients with glucocorticoid-induced osteoporosis (GCO) was evaluated. Twelve steroid-dependent asthmatic male patients with GCO were administered 200 mg of MPA (Depo-Provera) intramuscularly, and had fasting serum samples obtained at baseline and at weekly intervals for 5 consecutive weeks. Baseline serum samples were also obtained from 12 control healthy male subjects matched for age. The following measurements were made from each serum sample: osteocalcin (OC), skeletal (SAP) and total alkaline phosphatase (TAP), calcitonin (C), insulin-like growth factor I (IGF1), 1,25-dihydroxyvitamin D and 25-hydroxyvitamin D. Significantly lower baseline serum levels of OC and C were found in the patients with GCO than in controls (P less than 0.001). Following MPA administration in GCO patients statistically significant and sustained increases in OC, SAP and C were noticed during the next 5 weeks. No significant differences in baseline levels for TAP, IGF1, 1,25(OH)2D and 25(OH)D between GCO patients and controls were found, and no significant changes following MPA administration in GCO patients were obtained for these parameters. In conclusion, when administered to patients with GCO, MPA seems to stimulate the osteoblastic activity as suggested by sustained increases in OC and SAP serum levels, and also enhances the C production by the C-cells of the thyroid.