Seventeen cases are described in which both atria connect directly to a chamber with right ventricular characteristics. The atria connected through separate atrioventricular valves in six hearts and a common valve in 11. All hearts had a posterior rudimentary chamber. The septum which separated it from the main chamber was directed to the crux of the heart. Ten hearts were from patients with atrial situs solitus and seven from patients with atrial situs ambigous. Arterial connections were concordant in three cases, had a double outlet from the main ventricular chamber in nine and single outlet of the heart in five. The patent artery always arose from the main chamber, with pulmonary atresia in three and aortic atresia in two. This and other studies indicate that double inlet atrioventricular connection does not predict the morphology of the main chamber. Although usually associated with a main chamber of left ventricular type, it may also be associated with a main chamber having right ventricular characteristics. Both types should be considered as univentricular hearts; the posterior chamber in hearts of right ventricular type are analogous to the anterior chamber in univentricular hearts of left ventricular type and are a rudimentary chamber rather than a hypoplastic ventricle. In the right ventricular form of univentricular heart, the trabecular zone of the rudimentary chamber is of left ventricular type.