Congenital heart diseases were studied in children diagnosed of trisomy 13 and trisomy 18 in our hospital between January 1973 and July 1990. Twenty patients with trisomy 18 were diagnosed (18 females and two males). All had cardiac malformations. The findings were: ventricular septal defect in 16 cases (80%), valvular anomalies in 12 (63%), patent ductus arteriosus in nine (47%) and atrial septal defect or patent foramen ovale in 7 cases (36%). We found some complex congenital cardiac diseases: one atrioventricular canal, one tetralogy of Fallot, one hypoplastic left ventricle with mitral atresia and double outlet right ventricle, one case of univentricular heart with aortic outlet from a rudimentary cavity, a right ventricular atresia with pulmonary and tricuspid valves atresia. Nine cases of trisomy 13 were diagnosed (seven females and two males). We found: ventricular septal defect in 7 cases (77%), valvular disease in five (100% of the necropsy studies), secundum atrial septal defect in 4 patients (80%) and patent ductus arteriosus in two. Two cases presented hypoplastic left ventricle with aortic arch hypoplasia, one of them had subaortic stenosis and left superior vena cava being connected to the right atrium via coronary sinus; one case showed fibroelastosis. Our results have been similar to the previously reported and confirm the invariably presence of cardiac malformations in these syndromes. These malformations are an important sign of suspicion in fetal ultrasonography.