A 16-month-old girl developed severe pulmonary hypertension after closure of a large ventricular septal defect. All conventional therapeutic measures failed; an attempt to add nitric oxide at a continuous low dose to the inspired gas allowed resolution of pulmonary hypertension and low cardiac output. This report documents that continuous inhalation of low dose nitric oxide is capable of selective resolution of pulmonary hypertension following cardiac surgery for a large septal defect in a child. This suggests that a transient dysfunction in the release of nitric oxide by the pulmonary endothelial cell is responsible for the vasoconstriction.