Cardiac function was examined in 21 patients with Becker muscular dystrophy (BMD) and compared with 43 patients with Duchenne muscular dystrophy (DMD) and 37 healthy control subjects. Electrocardiography showed myocardial damage was most frequently found in the lateral wall, compatible with autopsy findings. The ratio of the preejection period to the ejection time was higher in patients with BMD (0.37 +/- 0.07, mean +/- SD) than in patients with DMD (0.28 +/- 0.05) and healthy controls (0.23 +/- 0.04). Left ventricular dimension and mitral annular size at end diastole in patients with BMD increased to 52.3 +/- 7.7 mm and 28.8 +/- 5.3 mm with age, respectively. In patients with cardiac failure and BMD, mitral regurgitation was observed at a rate of 66.7%. No definite relation between the deleted locus of the dystrophin gene and cardiac failure was found. Because motor dysfunction progresses more slowly in BMD than in DMD, a prolonged work load on the morbid myocardium may lead to dilated cardiomyopathy with mitral regurgitation.