Inhaled low-dose nitric oxide (2, 10, 20 ppm), together with high inspired oxygen concentration (0.80), was administered after corrective operations 13 times to 10 infants (median age 6 months) who were at risk of postoperative pulmonary hypertension because of their congenital heart disease and left-to-right shunt. Inhaled nitric oxide, even in a very low dose (2 ppm), caused selective pulmonary vasodilatation. The pulmonary/systemic artery pressure ratio was a predictor of the response to nitric oxide, with a greater response being seen in those with a high ratio (> or = 0.50). In children with a high pulmonary/systemic pressure ratio, the mean pulmonary vascular resistance index fell by 37% to 42%, accompanied by only a 10% fall in the systemic vascular resistance index but a 14% to 16% rise in mean cardiac index. No toxicity was seen in any subject. This exciting new therapy may prove to be an important adjunct in the management of postoperative pulmonary hypertension in the child with congenital heart disease.