The structure of the tricuspid valve in 14 cases of Ebstein's malformation is described and compared with that in the normal heart. The anomalous hearts showed a spectrum of malformation, varying from minimal displacement of the adjacent parts of the septal and inferior leaflets of the tricuspid valve to presence of an imperforate membrane or muscle shelf between the inlet and trabecular zones of the right ventricle. In "typical" Ebstein's anomaly the anterosuperior leaflet was abnormally attached so as to partially obstruct the route from right atrium to distal right ventricle, the communication between these chambers being between the abnormally attached leaflet and its malformed neighbors. In two hearts, a muscular shelf produced stenosis between inlet and trabecular portions, but a competent valve was produced at this site, the atrialized part of the right ventricle being of normal thickness. The significance of these findings is considered with regard to possible methods of repair. Description of the anomaly is facilitated by considering the right ventricle as possessing inlet, trabecular, and outlet portions. The essence of the anomaly is displacement of the tricuspid orifice to the junction of the inlet and trabecular ventricular zones.