Ascending aorta-pulmonary artery (Waterston) anastomosis was performed in 75 children, 51 of whom were younger than 1 month of age and 36 younger than 1 week of age at the time of operation. There were 21 operative and 8 late deaths. Operative deaths occurred more frequently in infants with complex cardiac conditions and severe hypoxemia and acidosis preoperatively. The postoperative status of the pulmonary arteries or arterioles could not be correlated with the degree of cardiomegaly, the pulmonary vascular markings, or characteristics of the shunt murmur. 64% of the patients catheterized had one or more postoperative structural abnormalities of the pulmonary arteries following the operation, most frequently kinking or narrowing of a pulmonary artery. Occlusion of a pulmonary artery occurred in 5 of 33 patients studied by angiography. Pulmonary vascular disease may also develop. Therefore, patients should be catheterized 1 year postoperatively to evaluate the level of pulmonary vascular resistance and the pulmonary arteries.