Cardiomyopathy is one of the most important clinical manifestations in Becker muscular dystrophy (BMD), but the severity of cardiac involvement is not usually related to that of skeletal muscle disability in this disease. In general, BMD patients who carry the mutations in the 5' end of the dystrophin gene tend to show severe cardiac dysfunction compared with skeletal myopathy, and these mutations may be responsible for more selective involvement of dystrophin appearance in the myocardium. In this article, we review the causative relationship of the variable clinical pictures and the dystrophin gene abnormalities in BMD patients, focusing on the severity of cardiac disorder.