Polymyositis (PM) is one of the inflammatory myopathies, disorders characterized pathologically by the presence of inflammatory infiltrates in striated muscle. The principal clinical manifestation of PM is proximal muscle weakness. The cause of PM is unknown, but current evidence suggests that it is an autoimmune disorder. PM can affect people of any age, but most commonly presents between the ages of 50 to 70. PM is rarely seen in people younger than 18 years of age, and is twice as common among females than males. PM is more common in blacks than in whites. The overall prevalence of PM is 1 per 100,000. Muscle weakness may develop suddenly or more insidiously over a period of weeks to months. The classic symptom of PM is proximal weakness, which may manifest as difficulty holding the arms over the head, climbing stairs, or rising from a chair. Weakness of the striated muscle of the upper esophagus may result in dysphagia, dysphonia, and aspiration. The chest wall muscles may be affected, leading to ventilatory compromises. Involvement of cardiac muscle may lead to arrhythmias and congestive heart failure. Dermatomyositis (DM) is closely related to PM, and both are distinguished primarily by the occurrence of characteristic skin abnormalities in the former. PM and DM may be associated with a variety of malignancies. PM may also occur as part of the spectrum of other rheumatic diseases like systemic lupus erythematosus and mixed connective tissue disease. Moreover, inflammatory myopathy may be caused by some drugs (procainamide, D-penicillamine), and viruses, most notably the retroviruses. Corticosteroids and immunosuppressive agents are the mainstays of therapy for PM. The principal goals of therapy are to improve strength and improve physical functioning. Many patients require treatment for several years. The 5-year survival rate for treated patients is in the order of 95%. Up to one-third of PM patients may be left with some degree of residual muscle weakness.