Based on the study of 149 case reports it appears that endocardial fibroelastosis is an entity which manifests itself in three distinctive types of clinical pattern. These have been designated fulminating, acute and chronic types of disease, according to the age group, type of onset, symptomatology and course of illness. Utilization of this clinical classification to make the differential diagnosis of idiopathic cardiomegaly is described. Pathologic findings are summarized and tabulated. Pathogenesis is speculative but developmental defect resulting from persistence and overgrowth of the primitive lining of the left bulbus cordis best explains the site and character of the lesion.