Deflazacort and prednisone were given to 26 patients with rheumatoid arthritis, polymyalgia rheumatica, or other chronic inflammatory diseases, in a double-blind study. Deflazacort rapidly and effectively suppressed disease activity in a manner supporting its assumed therapeutic potency of 83% that of prednisone. Prednisone induced a rapid increase in the level of daily calcium excretion that was not evident with deflazacort. Cortisol secretion was acutely inhibited by prednisone, but not by deflazacort. Neither corticosteroid had a significant effect on glucose metabolism, at the doses studied. Treatment with deflazacort may be an effective alternative to prednisone treatment, with fewer adverse effects on levels of calcium and cortisol, in patients with severe inflammatory conditions warranting the use of glucocorticoids.