The gross and microscopic appearance of the hearts from eight patients with Duchenne's progressive muscular dystrophy are described. Seven hearts had gross evidence of myocardial fibrosis, five of these demonstrating distinctive fibrosis of the epimyocardial portion of the free wall of the left ventricle, often with a striking band-like appearance. On the basis of mapping studies of the myocardial fibrosis, a theory regarding the progression of myocardial fibrosis in Duchenne's progressive muscular dystrophy is presented. Correlation of the pathologic anatomy, electrocardiograms, and vectorcardiograms in these patients and the family studies of others suggests that Duchenne's progressive muscular dystrophy represents a generalized cardiomyopathy that has its gravest and most distinctive effect on the epimyocardial portion of the free wall of the left ventricle.