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The purpose of this study was to determine the incidence, risk factors, and outcomes of acquired stenosis of the neopulmonary valve after the neonatal arterial switch operation.Reviewed were the preoperative and follow-up echocardiograms from 136 of 288 patients undergoing the arterial switch operation for whom adequate studies were available. Pulmonary stenosis was defined as a thickened and doming pulmonary valve and a pressure gradient of 20 mm Hg or more. Transposition of the great arteries was present with intact ventricular septum in 91 patients, with a ventricular septal detect in 39, with an aortic coarctation in 5, and with double-outlet right ventricle in 1 patient. No patient had preoperative valvular abnormalities (i.e., a bicuspid valve).During a median follow-up of 18 months (range <1 to 90 months), 32 patients (24%) had the development of supravalvular pulmonary stenosis, 15 (11%) with associated pulmonary valve stenosis (group I). Kaplan-Meier estimates of freedom from any intervention were 94% (95% confidence interval, 90% to 99%) at 1 year and 79% (95% confidence interval, 64% to 94%) at 5 years. The valve anulus before the arterial switch operation was significantly larger (p < 0.03) in those in whom neopulmonary valve stenosis did not develop (group II) than it was in those in whom it did (group I). At follow-up, the pulmonary valve anulus had decreased significantly in diameter in group I (p < 0.0005) and had remained larger in group II (p = 0.06) compared with normal diameter. Group I patients had the development of significant pulmonary valve hypoplasia (p < 0.03) whereas group II patients continued to have significantly larger valves compared with normal size (p < 0.0001).Neopulmonary valve stenosis after the arterial switch operation is not uncommon and is associated with growth failure of the valve anulus often associated with supravalvular pulmonary stenosis.