A 5-month-old female infant with Down syndrome underwent surgical repair of a complete atrioventricular septal defect and a patent ductus arteriosus. The postoperative course was complicated by life-threatening pulmonary hypertensive crises which did not respond to conventional vasodilating therapy. Nitric oxide inhalation resulted in rapid decrease in pulmonary artery pressure and improvement of haemodynamics and oxygenation without causing any untoward side effects. Inhalation of NO was continued over the next 4 days, with a maximum NO concentration of 20 ppm during a period of 12 hours. The subsequent postoperative course was uneventful and the patient was discharged from the hospital in good health on the 23rd postoperative day.