A 2-day-old female infant was presented with severe cyanosis without cardiac failure. The ECG revealed a normal axis and left ventricular hypertrophy. Chest X-ray examination showed a slightly but uncharacteristically enlarged heart. Cardiac catheterization and angiography showed isolated right ventricular hypoplasia with atrial septal defect. A large right to left shunt at atrial level produced marked systemic arterial desaturation. Up to the age of now nine months the patient remained in good health without cardiac failure. The clinical findings and surgical treatment of this rare malformation are discussed. Differential diagnosis with clinically similar malformations such as tricuspid atresia type 1 B or pulmonary atresia type 1 (with intact ventricular septum) is impossible without angiography.