OBJECTIVE: To evaluate the safety of intravenous (IV) sildenafil, an inhibitor of cyclic guanosine monophosphate-specific phosphodiesterase, in treating near-term and term newborns with persistent pulmonary hypertension of the newborn (PPHN). STUDY DESIGN: This was an open-label, dose-escalation trial in newborns with PPHN and an oxygenation index (OI) > 15. Sildenafil was delivered by continuous IV infusion for at least 48 hours and up to 7 days. RESULTS: Five centers enrolled a total of 36 neonates with PPHN at a mean of 34 +/- 17 hours of age; 29 of these neonates were already receiving inhaled nitric oxide (iNO). A significant improvement in OI (28.7 to 19.3; P = .0002) was observed after 4 hours of sildenafil infusion in the higher dose cohorts. Thirty-five neonates survived; 1 neonate required extracorporeal membrane oxygenation (ECMO) support. In 4 neonates, sildenafil was stopped due to adverse events. Seven neonates were enrolled before developing the need for iNO. In these neonates, OI improved significantly by 4 hours after initiation of sildenafil infusion (24.6 to 14.7; P = .009); 6 neonates completed treatment without the need for iNO or ECMO. CONCLUSIONS: IV sildenafil was well tolerated, and acute and sustained improvements in oxygenation were noted in those neonates who received the higher infusion doses.