Abstract The clinical, electrocardiographic, hemodynamic and cardiac pathologic features are described in two patients with the rapidly progressive, pseudohypertrophic, sexlinked form of Duchenne's muscular dystrophy. Previous studies have shown that the electrocardiogram is distinctive enough to aid in identifying this particular type of heredofamilial myopathic disorder even when the clinical features of the systemic myopathy are inconclusive. Tall right precordial R waves and deep limb lead and left precordial Q waves characterize these electrocardiograms. Information thus far has not provided explanations for either the distinctive morphology of the Duchenne electrocardiogram or for the variety of rhythm disturbances that have been observed. Accordingly, an electrocardiographic-pathologic correlative study was undertaken in order to determine whether a relationship existed between the distribution of myocardial dystrophy, the presence of an unusual type of small vessel coronary arteriopathy, and the electrocardiographic abnormalities of both QRS configuration and rhythm. The present study indicates that the anterior shift of the QRS (tall right precordial R waves) relates to a loss of posteriorly directed electrical forces caused by selective scarring of the posterobasal portion of the left ventricle. The abnormally deep Q waves appeared to reflect lateral extension of this scarring. These areas of myocardial fibrosis corresponded precisely to the locations of altered electrical activity demonstrated by the electrocardiogram. The reason for this unusual focal location of myocardial fibrosis was not determined. Although striking abnormalities were found in small intramural coronary arteries, the distribution of scarring was unrelated to the arteriopathy. The pathologic observations provided only scanty information regarding the rhythm disturbances. Degeneration of neurogenic fibers of the sinus or atrioventricular nodes could not be held responsible, but in one patient the arteries to both cardiac nodes were abnormal and may have contributed to the clinical arrhythmias.