Polymyositis is an inflammatory muscle disease preferentially affecting proximal muscles. Diagnosis is suggested by an EMG study showing abnormal spontaneous activity in proximal muscles and the presence of myopathic motor unit changes. Alternative diagnoses are eliminated by nerve conduction and neuromuscular junction studies and the EMG. Diagnosis is confirmed by muscle biopsy. Treatment requires aggressive and long term immunosuppressive therapy. Expectations for improvement in strength are good, but complete remission or cure less predictable. Polymyositis may be associated with other systemic disorders such as collagen vascular diseases and cancer, and an appropriate evaluation should include these associations.