Anatomical studies were made on 70 necropsied hearts with atrioventricular defects from patients with situs solitus and atrioventricular concordance, all having a common atrioventricular orifice. The arterial connections were concordant in 68 and were double outlet right ventricle in two; cases with arterial discordance (transposition) or single outlet of the heart were excluded. It proved possible to subdivide the hearts, depending on the morphology of the valve leaflets. Five leaflets were distinguished by the commissural pattern and their insertion to major papillary muscles. They were a posterior bridging leaflet, right and left lateral leaflets, and right and left anterior leaflets. Subdivision was made on the basis of the disposition of the anterior leaflets. In six hearts the left anterior leaflet was committed to the left ventricle and the right anterior leaflet to the right ventricle, the commissure between them being on the crest of the ventricular septum. In 39 hearts there was minimal bridging of the left anterior leaflet so that it extended between the anterior papillary muscle of the left ventricle and the medial papillary complex of the right ventricle. In eight hearts the right margin of the left anterior leaflet was attached to an apical papillary muscle, while in 17 hearts it was attached to the anterolateral papillary muscle of the right ventricle. As the bridging of the left anterior leaflet increased, so the size of the right anterior leaflet decreased, but in all hearts both leaflets were identified. These findings were compared with previous classifications of complete atrioventricular defects.