This study was designed to investigate if the addition of Remodulin reduced the rate of clinical worsening (defined as the need for additional treatment targeting PPHN, need for extracorporeal mechanical oxygenation \[ECMO\], or death) in neonatal subjects with PPHN who did not show an adequate response to inhaled nitric oxide (iNO). This study was part of a pediatric investigation plan agreed upon by the EMA (EMEA 000207-PIP01-08-M08).