Persistent pulmonary hypertension in the newborn (PPHN) is characterised by increased medial and adventitial thickness in the lung vasculature. This study describes morphometry of lung vasculature after extracorporeal membrane oxygenation (ECMO) in newborns with PPHN, due to meconium aspiration syndrome, sepsis or idiopathic persistent pulmonary hypertension of the newborn (i-PPHN). Three groups were studied: newborns with PPHN treated with ECMO (n=9), newborns with PPHN not treated with ECMO (n=12) and age-matched controls without PPHN (n=11). In pulmonary arteries with an external diameter of less than 150 microm, arterial media, adventitia and total wall thickness, expressed as a percentage of the external diameter, and their cross-sectional areas were calculated. Newborns with PPHN, compared with controls, demonstrated increased percentage of media thickness, adventitia thickness and total wall thickness, and increased medial, adventitial and total wall cross-sectional area. Newborns treated with ECMO, compared with those not treated so, showed a decreased percentage of media thickness and medial cross-sectional area in arteries with an external diameter less than 75 microm, and decreased percentage of media thickness and decreased medial, adventitial and total wall cross-sectional area in arteries with an external diameter of 75-150 microm. ECMO for persistent PPHN, due to meconium aspiration syndrome, sepsis or i-PPHN, reduces the abnormal morphometry of small pulmonary arteries. The underlying mechanisms contributing to this improved morphometry are yet unknown.