Pulmonary hypoplasia (PH), secondary to premature rupture of fetal membranes (PROM), is a frequent cause of pulmonary insufficiency in preterm infants. Pulmonary hypoplasia of diverse causes is usually associated with pulmonary hypertension. The objective of this study was to quantitate the acinar arterial wall thickness and muscularization in preterm infants with PROM that died with PH in comparison to age-matched controls. The left lung and pulmonary vasculature were perfused and fixed in a standard method in 16 infants with PROM and PH, and in 16 controls. In infants with PH, the alveolar ductal artery walls were nearly twice as thick as those of controls, 15.1 +/- 5.1% versus 7.4 +/- 1.3%, p < 0.0001. Ninety-one percent had muscle in the wall compared to 18% of controls. We conclude that infants with PROM and PH have increased pulmonary arterial muscularization, placing them at significant risk to have pulmonary hypertension. For these infants, therapy to reduce pulmonary vascular resistance should be instituted early.