Polymyositis often is associated with mild sclerodermatous skin changes or transitory articular symptoms, and a malignant tumor coexists in a significant percentage of cases in adults. The primary feature is muscular weakness, involving especially the proximal girdle muscles and the swallowing mechanism; muscular pain and tenderness may be absent. The course varies from acute and fulminating to insidious and chronically progressive, and seems to be more aggressive in children. Serum enzyme determinations, electromyography and muscle biopsy are the most reliable diagnostic aids. Corticosteroids have proved beneficial in almost all cases. They are only temporarily helpful when malignant tumor coexists, but other patients treated early have regained almost normal strength. Low-dosage maintenance therapy usually has been necessary.