Two-year-old boy with Down's syndrome was treated with inhaled nitric oxide for pulmonary hypertensive crisis after correction of complete atrioventricular canal and persistent ductus arteriosus. The acute rise in pulmonary pressure and hemodynamic instability occurred on 3rd, 4th and 6th post operative days and pulmonary vasodilation with prostaglandin E1 and amrinone had failed. Twenty ppm of inhaled nitric oxide effectively reduced the pulmonary pressure and the patient was subsequently weaned from ventilation. Inhaled nitric oxide exerted a pulmonary vasodilation without decreasing systemic pressure in this patient with postoperative pulmonary hypertensive crisis.