Although the effects of inhaled Nitric Oxide on pulmonary vascular tone are well-described and relevant to term infants with persistent pulmonary hypertension, the pathophysiology of respiratory failure in preterm infants may be quite different. Chronic lung disease (CLD) represents the final pathway of a heterogeneous group of pulmonary disorders of infancy that usually start in the neonatal period. CLD most commonly occurs in preterm (<30 weeks of gestational age (GA) infants with birth weights less than 1,500 g, and especially in those very preterm (<26 wks GA) with birth weights less than 1,000 g, and who have been treated for respiratory distress syndrome (RDS).