IN OUR experience the cardiac lesion usually known as endocardial sclerosis is one of the commonest types of fatal heart disease of infancy. The condition, however, is not well known, and the symptoms have not been emphasized in most textbooks. It is also evident that the multiplicity of names attached to the disease in medical reports has led to confusion. In early descriptions the condition was called "fetal endocarditis" or was included under the title of "congenital idiopathic hypertrophy" of the heart. Recently, such terms as endocardial fibrosis, prenatal fibroelastosis, elastic tissue hyperplasia, endocardial sclerosis, and others have been used to describe what is apparently a single pathologic condition. Since the cause and the exact nature of the disease are still obscure, it is difficult to select a title which is satisfactory. The name endocardial sclerosis is employed here because it describes fairly accurately and briefly the gross appearance of