An unusual form of atrioventricular septal defect was found at operation. The anatomy was that of a primum atrial septal defect with deviation of the atrial septum to the left. Thus from the right atrium both right and left atrioventricular valves could be seen. The left-sided valve was tricuspid. Repair was achieved by closure of the septal commissure of the left-sided valve ("cleft"), excision of the lower part of the atrial septum, and insertion of a pericardial patch. The lower part of the patch was sutured between the left and right atrioventricular valves. A specimen with similar anatomy is also described.