Echocardiographic studies were performed on six patients with autopsy-proved aortic atresia and hypoplastic left ventricle and two patients with surgically proved tricuspid atresia and hypoplastic right ventricle. The findings were compared to those from the echograms of 50 normal newborns who served as a control group. The ultrasound measurements obtained in the normal newborns were as follows: mean right ventricular end-diastolic dimension, 1.3 cm (range, 1.0 to 1.7), mean left ventricular end-diastolic dimension, 1.6 cm (range, 1.2 to 2.0), mean left atrial dimension, 0.9 cm (range, 0.6 to 1.3), and mean left ventricular outflow tract dimension, 1.0 cm (range, 0.7 to 1.2). The mean velocities of closure of the mitral and tricuspid valves during passive filling of the ventricles were 53 mm/sec and 43 mm/sec. The ranges were 36 to 80 mm/sec for the mitral valve and 34 to 56 mm/sec for the tricuspid valve.In the patients with aortic atresia, the significant findings were a larger than normal right ventricular chamber (mean, 2.5 cm), a left ventricular chamber measuring less than 0.9 cm, and an absent or grossly distorted mitral valve echo. By contrast the patients with tricuspid atresia had very small right ventricular dimensions less than 0.6 cm, no demonstrable tricuspid valve echo, and a larger than normal left ventricle (mean, 2.3 cm). The above findings were diagnostic and were not confused with the normal newborn or other forms of congenital heart disease.