Dermatomyositis and polymyositis are diseases which must be carefully defined in each patient. Criteria such as those suggested by Bohan and Peter will help define the disease and exclude other possible causes of myositis. Immunological factors are probably involved in the pathogenesis of the disease, although varied types of injury may manifest in clinically similar disease. Dermatomyositis, not polymyositis, is related to internal malignancy in adults; however, an extensive malignancy evaluation is not indicated. Therapy with corticosteroids will improve the quality of life, will prevent contractures and calcinosis and may improve the chance of survival. In cases where steroids are ineffective, immunosuppressives may be helpful.