Four patients with polymyositis were treated with azathioprine. Each had been treated initially with prednisone and had become unresponsive or suffered severe complications of steroid therapy. Response to azathioprine therapy, as measured by improvement in muscle strength and decrease in serum enzyme levels, was favorable in all patients. Treatment with azathioprine permitted decrease in prednisone dosage with resultant resolution of steroid side effects. No important complications of azathioprine therapy were encountered. We advocate the use of azathioprine in the treatment of polymyositis.