Congenital heart disease occurs in more than 40% of patients with Down syndrome. Malformations include atrioventricular septal defects and approximately 33% of all atrioventricular septal defects are associated with Down syndrome. This congenital defect is referred to as an endocardial cushion defect or atrioventricular canal malformation which results from a defective closure of the endocardial cushions. These are a group of abnor¬malities unified because of a complete absence of normal atrioventricular septal structures. This is a much more complicated lesion than the more common atrial septal defect and is much more difficult to repair surgically. For more information about atrial septal defects see the chapters Atrial Septal Defect and Embryology. To learn more about ventricular septal defects see the chapter Congenital Heart Disease.