A total of 793 premature newborns will be enrolled from 14 centers within 48 hours of birth. They will be randomly assigned to receive either placebo or iNO at 5 ppm until the breathing tube can be safely removed or after 21 days. The iNO will be delivered by an INOvent delivery system in such a way that physicians and nurses will not know which treatment each participant is receiving. Management strategies for aspects of patient care including mechanical ventilation, surfactant administration, fluid administration, and steroid use will be determined by physicians at each center. Serial cranial ultrasounds and methemoglobin levels will be monitored to determine adverse events. The first 200 patients will have serial blood samples and tracheal aspirates obtained for measurements of inflammatory mediators, including interleukin-6 (IL-6), interleukin-8 (IL-8), interleukin-10 (IL-10), endothelin-1, myeloperoxidase, neutrophil counts (tracheal aspirates), and endothelin-1 (blood). Participants will be seen at 12 and 24 months of age to monitor the long-term effects on the cardiopulmonary or neurologic systems. At these visits, a health questionnaire will be administered and Bayley II scales of infant development will be completed.