Eighty cases of Down's syndrome were found among 1504 children with congenital heart disease under the age of 15 years. The most common cardiac anomaly, ventricular septal defect, was found in 49 per cent of the 80 cases studied, while the second most frequently encountered anomaly, common atrioventricular canal, was found in 15%. Haemodynamic investigations of 24 cases of ventricular septal defect showed Eisenmenger's syndrome to be present in 10 cases; this seemed to appear at an earlier age in mongoloid children than in other children with ventricular septal defect. Right-to-left shunt was found in 5 out of 9 cases of common atrioventricular canal. Pulmonary hypertension was found in all of 24 cases of ventricular septal defect and in 7 of 9 cases of common atrioventricular canal. The cumulative survival up to 10 years was 64% for girls and 49% for boys. Death was most commonly the result of pulmonary complications which occurred in 22 out of the 34 patients who died.