Background: Most patients with progressive muscular dystrophy of the Duchenne type (DMD) die of respiratory failure at approximately 20 years of age. However, some patients with DMD die of heart failure in relatively short periods of time. We investigated the long-term progression of cardiac impairment in patients with DMD by two-dimensional echocardiography. Methods: We monitored 74 patients for 4 years with two-dimensional echocardiography. Patients were classified into four groups according to the 8-grade Swinyard-Dever system. We also evaluated the echocardiographic features of 22 other patients with DMD with studies performed within 1 year before their deaths. Results: During the 4-year follow-up the left ventricle expanded, and regional left ventricular wall motion abnormalities developed in the posterior wall and apex. Almost all patients had myocardial dysfunction that progressed in parallel with their Swinyard-Dever stage. However, in a few patients who died of congestive heart failure, left ventricular dilation and circumferential left ventricular wall motion were severely impaired. Conclusions: Myocardial impairment is accelerated in patients with DMD who died of heart failure. Two-dimensional echocardiography is a useful tool for the early diagnosis of left ventricular dysfunction and provides useful information for the treatment of patients with DMD. (Am Heart J 1998;135:937-44.)