The criteria for the diagnosis of polymyositis, the differential diagnosis, and treatment are reported from the Division of Rheumatology and Internal Medicine, Mayo Clinic, Rochester, MN. A muscle biopsy is considered the standard diagnostic procedure for polymyositis but it is not infallible. Characteristic features are inflammatory mononuclear cell infiltration, degenerating and regenerating muscle fibers, and central nuclei. Electromyographic findings are characteristic but not specific. They include insertional activity, fibrillation potentials, motor unit potentials of increased frequency and decreased duration, and normal conduction velocity in nerves. The most useful blood test in polymyositis is a serum CK determination but this may be normal in about 2% of patients. The MM fraction constitutes the majority of the CK in polymyositis, the MB fraction may be increased because of skeletal muscle regeneration and less often because of myocarditis. Treatment with high dose corticosteroids is administered as early as possible in the course of the disease. The longer the delay before diagnosis and appropriate treatment, the worse the prognosis. The best known dermatological change is Gottron’s sign, an erythema over the knuckles. Gottron’s rash is a scaly, violaceous rash on various areas of the body including the hands, elbows, and knees as well as a heliotrope rash on the eyelids. Calcinosis occurs in 13% of childhood cases but in less than 5% of adult cases of polymyositis. No therapy has proved effective for calcinosis and occasionally, surgical excision of painful calcific areas has been necessary. When polymyositis develops during pregnancy the incidence of fetal loss is 32%. Women with a history of inactive polymyositis should be warned that pregnancy might exacerbate the disease. Myositis has been reported with human immune deficiency virus infection and may closely mimic the myopathic changes seen in polymyositis.