A clinical study of 30 patients with rheumatoid arthritis was undertaken in order to assess the acute effects of corticosteroids on calcium and bone metabolism. The patients were randomly divided into 3 groups. The first group was not treated with corticosteroids, the second group was treated with 3 oral pulses of 100 mg prednisolone and the third group received 3 intravenous pulses of 1000 mg methylprednisolone (MP) on alternate days during one week. In both steroid treated groups the serum parathyroid hormone concentration tended to increase. In the MP treated group an increase in the 1,25-dihydroxyvitamin D concentration after the first pulse was followed by a significant drop; this effect was also seen, but somewhat retarded and less distinct, in the orally treated group. In the MP treated group the urinary calcium excretion raised significantly 6 hrs after the first pulse and then dropped significantly. In all groups no changes were found in the serum calcium level and the urinary excretion of hydroxyproline. We conclude that, acute changes in calcium and bone metabolism occur during treatment with intravenous pulses of methylprednisolone and with oral pulses of prednisolone. These changes are small and reversible in a few days.