Down syndrome remains the most common chromosomal abnormality in live-born infants in the world today. The association between Down syndrome and congenital heart disease (CHD) is well known, and it is widely recognized that CHD contributes significantly to the morbidity of children with Down syndrome. The reported incidence of CHD in Down syndrome patients is between 40 and 60%. The most commonly described defect is complete atrioventricular septal defect (AVSD), which comprises 30–40% of all cardiac defects. Complex genetic factors are involved. Routine cardiac screening of all newborn babies with Down syndrome is recommended. Expert groups suggest that the cardiac status of all children with Down syndrome should be established by 6 weeks of age to permit appropriate and timely treatment avoiding the establishment of irreversible pulmonary vascular disease that would make corrective surgery impossible.