Wilson's disease is a multisystem disorder. Heart involvement in Wilson's disease, however, has rarely been recognized. A prospective study was undertaken of 53 consecutive patients (28 men and 25 women, mean age of 21.4 years) with Wilson's disease. Electrocardiographic abnormalities occurred in 18 of 53 patients (34 percent), including left ventricular hypertrophy, biventricular hypertrophy, early repolarization, ST depression and T inversion, premature atrial or ventricular contractions, atrial fibrillation, sino-atrial block, Mobitz type 1 atrioventricular block, and tremor artifact. In contrast, 26 medical students and 14 carriers of Wilson's disease as control subjects (mean age of 22.6 years) all showed normal ECG. Eight out of 43 patients (19 percent) demonstrated asymptomatic orthostatic hypotension. An abnormal response to the Valsalva maneuver occurred in six of 18 patients (33 percent). There were two cardiac deaths; one died of repeated ventricular fibrillation (the copper content in the myocardium was 2.28 micrograms/g, and in the bundle of His 1.21 micrograms/g wet weight in the autopsy specimen); and the other, of dilated cardiomyopathy. It is concluded that four modes of cardiac manifestations in Wilson's disease include arrhythmias, cardiomyopathy, cardiac death, and autonomic dysfunction. Such possible cardiac involvement should be added to the clinical picture of Wilson's disease involving the hepatic and central nervous system.