Bronchopulmonary dysplasia (BPD) is a disease of chronic respiratory insufficiency stemming from premature birth and iatrogenic lung injury leading to many complications. BPD is the most common pulmonary sequela of prematurity and is often fatal; however, there remains no FDA-approved therapies to treat or prevent BPD. Sildenafil is increasingly used off-label in premature infants despite scant safety and efficacy data. We developed the phase II SIL02 trial to describe the safety, pharmacokinetics and preliminary effectiveness of intravenous and enteral sildenafil in premature infants at risk for BPD.