The morphological characteristics of mitral atresia were studied in 30 hearts to determine the presence or absence of a morphological rudiment of the atretic valve and the relation of this rudiment, if found, to any chamber in the ventricular mass. All the hearts showed atrial situs solitus and no ventricular inversion; consequently all had left atrioventricular atresia. In all instances dense fibrous tissue connected the floor of the left atrium to the left ventricle. This connective tissue is considered to be the morphological rudiment of the atretic mitral valve. In several hearts the intervening fibrous tissue varied from a thick fibrous membrane to a tiny fibrous cord; it is impossible to detect these variations clinically. It is, therefore, more practical to classify those hearts which have a detectable fibrous membrane macroscopically as having an "imperforate membrane" and those with a fibrous strand detectable only microscopically as having an "absent atrioventricular connection."