Infants were evaluated at baseline for pulmonary, cardiac, bleeding status, and therapies received. Those who were greater than 34 wks gestation and 14 days old or less and required assisted ventilation with an OI (mean airway pressure x FiO2 divided by the PaO2 x 100) greater than 25 were eligible. They were randomly assigned to receive iNO at 20 ppm or 100 percent oxygen as a control. Infants whose PaO2 increased by less than 20 mm Hg after 30 minutes were studied at 80ppm iNO or control gas.; MetHg and NO2 concentrations were monitored regularly. Management, including surfactant administration, included prospectively defined criteria for study gas response, escalation, reinitiation, and weaning. The maximum total time on study gas was 336 hrs (after 240 hrs, the INO concentration was required to be no more than 5 ppm). Patients were followed to death, discharge, or 120 days and evaluated at 18-24 mos by a masked certified examiner.