Six cases of heart disease have been presented, with a hint that they may be type specimens for a distinct species of congenital cardiac malformation. They are representative of a syndrome characterized pathologically by hypertrophy of the left side of the heart, thickening of the mural endocardium of the left ventricle, and varying distortion of the aortic and mitral valves, and characterized clinically by radiologically demonstrable cardiac enlargement, nondiagnostic murmurs and sudden death in infancy. The older concept of an inflammatory pathogenesis of so-called "fetal endocarditis" is rejected in favor of developmental origin of the lesions.