AMONG CAUSES of cardiac enlargement Of obscure origin that confront the clinician and pathologist alike are subendocardial fibroelastosis, inflammatory myocardiopathies, glycogen-storage disease, amyloidosis, muscular subaortic stenosis,' asymmetrical hypertrophy of the heart,2 idiopathic myocardiopathy,3 and familial cardiomegaly. Familial cardiomegaly, originally named in 1949 by Evans,4 has been the subject of many communications, primarily in the European literature, but it is little better understood now than after the original report. Various theories to explain the cardiomegaly include cardiac involvement with toxoplasma5 and trypanosomiasis, a myocardiopathy similar to that seen in Friedreich ataxia but without neurologic involvement,4 glycogen-storage disease, and an inheritable disorder of the myocardium transmitted as a Mendelian dominant,6 with sex linkage. Similar to the enigma of causation, the question of the specificity of the clinical picture and of the autopsy findings has been controversial with a familial occurrence being the only unequivocal common denominator. Unfortunately, relatively little emphasis has been devoted to the character and specificity of the morphologic findings, with the majority of reports having appeared in clinical literature. It is the purpose of this paper to present the clinical and necropsy studies of two sisters who died in young adulthood of progressive cardiac disease. Family history and the significance of specific anatomic findings are discussed with a few remarks regarding etiology.