In 6 patients with dystrophin-verified Becker muscular dystrophy (BMD), 3 patients had dilated cardiomyopathy (DCM group). The other 3 patients (non-DCM group) also had ECG abnormalities including incomplete right bundle branch block, left ventricular enlargement and intraventricular conduction defect. Between DCM and non-DCM group, there was no prominent difference in ages at onset, mean duration and severity of muscular weakness. Serum CK levels, and molecular weight and amount of dystrophin also showed no significant difference between two groups. On reviewing 14 BMD patients, including 3 present patients with cardiomyopathy, the cardiac symptoms appeared from 4 to 41 years, averaging 17.1 years of age. The mean duration of muscle symptoms was 9 years, ranging from 0 to 33 years. There was no correlation between severity of muscle weakness and cardiomyopathy. Six patients died of heart failure and 3 received cardiac transplantation. Thus there was no characteristic clinical feature in BMD patients with cardiomyopathy except for very poor prognosis. Since the myocardial involvement is not related with clinical severity and duration of the disease, careful observation for cardiac function should be carried out in all BMD patients even in the early stage of muscle weakness.