We reviewed the clinical records of 16 patients with polymyositis-dermatomyositis syndromes autopsied at The Johns Hopkins Hospital to determine the nature and extent of cardiac involvement and its correlation with the severity of disease as a whole. The adult patients ranged in age from 32 to 84 years (average 56); the 2 children were aged 2 and 10 years. The duration of disease ranged from 1 to 72 months (average 21). Seven patients had dermatomyositis, 5 had dermatomyositis with malignancy, 2 had childhood dermatomyositis, and 2 had an overlap syndrome. Seven patients had clinical evidence of congestive heart failure, 4 of whom had microscopic evidence of myocarditis. Two patients had bundle branch block; in 1 there was direct involvement of the conduction system by myositis and contraction band necrosis. Evidence of active myocarditis was present in 4 patients (25%); all had congestive failure. Focal myocardial fibrosis was present in 4 patients. Vascular alterations were present in the coronary arteries in 5 patients (31%). Three had active vasculitis, 1 had intimal proliferation, and 1 had medical sclerosis with calcification. All patients with active myocarditis had skeletal muscle involvement. Nine patients had myositis without myocarditis. There was no correlation of overall severity of the disease with the presence or absence of active myocarditis. The present study shows that cardiac involvement may be common in polymyositis; congestive failure or conduction abnormalities arising in this setting may be indicative of myocarditis.