Thirty-five patients with idiopathic inflammatory myopathy (IM) were treated with plasmapheresis. They had had inadequate clinical responses to prednisone or to prednisone and cytotoxic immunosuppressive drug therapy. All patients received cyclophosphamide or chlorambucil in addition to plasmapheresis; 33 also received prednisone. Strength increased in 32 patients during combined therapy; patients with the most active disease experienced substantial improvement that approached clinical remission in some cases. Clinical improvement was documented by muscle testing, forced vital capacity, electromyography, muscle biopsy, and serum creatine phosphokinase activity. The major side effect of treatment was herpes zoster. Primary biliary cirrhosis improved in one patient. Although the effects of plasmapheresis cannot be dissociated from those of the concomitantly administered immunosuppressive drugs, the improvement seen in patients with active disease suggests that plasmapheresis with immunosuppressive drug therapy may find a place in their treatment and implicates circulating factors in the pathogenesis of IM.