Twenty-one patients with polymyositis were prospectively examined with echocardiography, phonocardiography and electrocardiography. Cardiac performance, estimated with echocardiography, was enhanced as shown by a significant (P less than 0.01) increase in ejection phase indexes of left ventricular function compared with values in a matched control group. Known causes of the high output state, such as anemia or thyrotoxicosis, were not clinically evident. There was no evidence of left ventricular enlargement, left ventricular wall hypertrophy, or left atrial enlargement in the echocardiogram or chest X-ray film. The echocardiogram showed systolic mitral valve prolapse in 11 of 17 patients (65 percent) with an adequately imaged mitral valve; midsystolic clicks were present in 7 of these. One patient, who did not have prolapse, had echocardiographic evidence of a small pericardial effusion. Electrocardiographic abnormalities were present in 11 of 21 patients (52 percent) and included evidence of atrioventricular conduction disturbances, atrial and ventricular arrhythmias and left atrial abnormality. The pathophysiology of mitral valve prolapse and increased systolic left ventricular function in polymyositis remains uncertain; however, the spectrum of cardiac abnormalities, detected noninvasively in 16 of 21 of our patients (76 percent) may represent a high frequency rate of cardiac involvement in this disease.