ENDOCARDIAL fibroelastosis is a cardiomyopathy of unknown origin that affects infants and young children. It is characterized by poor myocardial contractility with hypertrophy and dilatation of the heart, especially of the left ventricle. At autopsy, a milky-white thickening of the endocardium is found.¹ Because similar endocardial thickening is seen in congenital cardiac anomalies complicated by subendocardial ischemia (e.g., aortic atresia), it has been suggested that endocardial fibroelastosis may be related to myocardial hypoxia.² Familial cases suggest that the disease may result from a genetically transmitted abnormality affecting cardiac metabolism.³ The cardiac tissue in congestive cardiomyopathy contains increased numbers of mitochondria, . . .