Case histories of four infants with single ventricle and left atrioventricular (AV) valve atresia, with interatrial obstruction but without pulmonic stenosis, are presented. Relief of interatrial obstruction by surgical or balloon atrial septostomy resulted in a marked fall in the left atrial pressure and the pulmonary artery mean pressure and resistance as well as a marked increase in the left-to-right interatrial shunt, pulmonary blood flow, and systemic arterial oxygen saturation. Because of this predictable fall in pulmonary vascular resistance, concomitant pulmonary artery banding was performed in each case with good results. There are only a few previously reported cases of single ventricle with left AV valve atresia. Interatrial obstruction is quite common in this lesion. The concept that relief of interatrial obstruction produces a rapid and predictable fall in the pulmonary vascular resistance and that concomitant pulmonary artery banding is necessary in these infants with complex congenital heart disease is emphasized.