Malfunctioning of the left atrioventricular valve has always been, and remains, the major incremental risk factor in the repair of atrioventricular septal defect. Now that the cardiac surgeon has ample time to assess the anatomy and function of the left valve, results have improved, but are still less than ideal. On the presumption that the anterior leaflet of the mitral valve is “cleft” in this anomaly, it used to be common practice to close the “cleft”. Currently, a substantial number of surgeons employ this technique, often irrespective of the individual anatomy, and in the majority of cases with success.