Persistent pulmonary hypertension of the newborn (PPHN) is a clinical disorder characterized by abnormal vascular structure, growth, and reactivity. Disruption of vascular growth during early postnatal lung development impairs alveolarization, and newborns with lung hypoplasia often have severe pulmonary hypertension. To determine whether pulmonary hypertension can directly impair vascular growth and alveolarization in the fetus, we studied the effects of chronic intrauterine pulmonary hypertension on lung growth in fetal lambs. We performed surgery, which included partial constriction of the ductus arteriosus to induce pulmonary hypertension (PPHN, n=14) or sham surgery (controls, n=13) in fetal lambs at 112-125 days (term=147 days). Tissues were harvested near term, for measurement of right ventricular hypertrophy (RVH), radial alveolar counts (RAC), mean linear intercepts (MLI), wall thickness and vessel density of small pulmonary arteries. Chronic DA constriction caused RVH (p<0.0001), increased wall thickness of small pulmonary arteries (p<0.002), and reduced small pulmonary artery density (p<0.005). PH also reduced alveolarization, causing a 27% reduction in RAC (p<0.005) and 20% increase in MLI (p<0.05). Furthermore, prolonged DA constriction (21 days) not only decreased RAC (p<0.0001) and increased MLI (p<0.05) by 30%, but also caused a 25% reduction of lung/body weight ratio (p<0.05). We conclude that chronic pulmonary hypertension reduces pulmonary arterial growth, decreases alveolar complexity, and impairs lung growth. We speculate that chronic hypertension impairs vascular growth, which disrupts critical signaling pathways regulating lung vascular and alveolar development, thereby interfering with alveolarization and ultimately resulting in lung hypoplasia.