A 32-year-old man with Duchenne muscular dystrophy (DMD) was admitted to the hospital because of worsening dyspnea and general fatigue. He had received medication therapy for cardiomyopathy with heart failure and home mechanical ventilation for respiratory failure. An electrocardiogram on admission showed intermittent third-degree atrioventricular block. Echocardiography showed global mild left ventricular systolic dysfunction with dyssynchrony (ejection fraction: 45%). He underwent implantation of a cardiac resynchronization therapy-defibrillator. His B-type natriuretic peptide level was improved after cardiac resynchronization therapy-defibrillator implantation, and he remains asymptomatic. The incidence of cardiomyopathy increases with age. By adulthood, 100% of patients have cardiac involvement.