Prospective echocardiographic diagnosis of absence of the left atrioventricular connexion, with the right atrium connected to a morphologic left ventricle through a bileaflet morphologically mitral valve, was made in six infants. The rudimentary right ventricle was left-sided in all patients, and separated from the left atrium by sulcus tissue. The ventriculoarterial connexions were discordant. Associated defects included subpulmonary stenosis (2 patients), pulmonary atresia (1 patient), and a patent duct (4 patients). All patients developed early left atrial hypertension due to a restrictive interatrial septum, and required transcatheter septostomy (5 patients), or surgical septectomy (3 patients). One patient who had a severely restrictive ventricular septal defect died following cardiac catheterization. In three others the ventricular septal defect has become progressively restrictive on serial catheterization. Successful intermediate term palliation has been performed in two patients using a bidirectional Glenn anastomosis, together with enlargement of the ventricular septal defect and a Damus-Kay-Stansel procedure in one. It is possible to distinguish this malformation from "mitral atresia" using cross-sectional echocardiography. The long-term outlook is influenced by early relief of left atrial hypertension. Balloon atrial septostomy alone is usually inadequate, and either blade septostomy or surgical septectomy are required. Serial cardiac catheterization is mandatory for planning definitive palliation.