We observed clinical response to inhaled nitric oxide (iNO) in 12 neonates with persistent pulmonary hypertension of the newborn (PPHN). Clinical response was defined as a decrease in oxygenation index (OI) by 40%. Ten of 12 neonates had response to iNO showing decrease OI from 46.1±7.6 to 14.4±6.8 at 1 hour after inhalation. Sustained improvement of OI was achieved in 8 neonates and two neonates were relapsed. In the group of neonates who had OI above 40 (n=7), 6 of them showed the decrease of OI from 66.1±4.8 to 18.3±8.0 at 1 hour. In two groups, one had OI of 40 or greater, and the other OI of 40 or less, there were no differences in pattern of response and early death rate. The response rates according to underlying diseases were as follows; idiopathic PPHN 100%, respiratory distress syndrome 100%, and diaphragmatic hernia 66.7%. Relapse was observed in one neonate with sepsis caused by pneumonia and in one infant with meconium aspiration syndrome. Two infants showed no response to iNO (one diaphragmatic hernia and one suspected pulmonary hypoplasia). We conclude that iNO therapy could improve oxygenation in high percentage of newborn infants with severe PPHN of various underlying conditions except pulmonary hypoplasia.