The causes of variable responsiveness to inhaled nitric oxide (NO) in Persistent Pulmonary Hypertension of the Newborn (PPHN) are unknown. The changes in the severity of respiratory failure after the onset of inhaled NO (maximal dose 20 ppm) were studied in 13 consecutive neonates with severe PPHN. Response was defined as a sustained decrease of alveolar-arterial oxygen gradient (AaDO2) by > 20%, or a decrease in oxygenation index (OI) by > 40%. Six neonates had a rapid response within 30 min, three had an intermediate response within 8 h, and three had a delayed response within 12 h after the onset of NO. Three infants with birth asphyxia responded rapidly to inhaled NO. One infant with sepsis did not respond, and two with suspected sepsis had a delayed response. The infants with Meconium Aspiration Syndrome and idiopathic PPHN had a variable response time. Twelve neonates required 4 to 14 days of mechanical ventilation and survived. Infants with PPHN may benefit from a trial of inhaled NO therapy that exceeds 30 min. The variability of the response time to inhaled NO is likely to be multifactorial and dependent on the disease process associated with PPHN.