The most typical clinical features of polymyositis (PM), the criteria of diagnosis and principles of treatment are outlined. An inflammatory disease of muscle, PM also frequently affects other organs such as the skin and hence the name dermatomyositis. The principal cardiac symptom is a peculiar disturbance of atrioventricular conduction, correlated with a specific anti-Ro autoantibody, present in 25% of patients. The etiology of PM is as yet unknown, although there is evidence for an autoimmune pathogenesis. It is frequently found in association with other immune-mediated diseases such as myasthenia gravis, pemphigus, immune-complex vasculitis and Sjogren syndrome. Laboratory investigations show hypergammaglobulinemia, a decrease of complement factors C3 and C4 and the presence of circulating immune complexes in 70% of patients. Very frequent, especially in cases of dermatomyositis, is a histologically detectable accumulation of IgG and complement in the walls of the intramuscular venous vessels. Cell-mediated hypersensitivity, emphasised formerly as highly significant in PM, has not been confirmed. The presence of specific antimyoglobin lymphocyto-toxicity, once considered to be the hallmark of muscle degeneration in PM, has been excluded by a number of laboratories. In a personal series of patients with various clinical forms of PM a severe loss of suppressor/cytotoxic lymphocytes was found in the peripheral blood and a relative increase in the first subset. These results support the hypothesis that a serious disturbance of immunoregulation is present in PM and is the cause of a multitude of immunological anomalies, the characterisation of which is under study.