Congenital diaphragmatic hernia (CDH) and congenital pulmonary airway malformation (CPAM) are diseases in which chest-occupying lesions can result in severe pulmonary hypoplasia. However, significant postnatal mortality due to pulmonary hypertension (PH) is more often seen in patients with CDH. We analyzed prenatal echocardiographic parameters of pulmonary vascular pathology in these groups to understand whether PH in patients with CDH is secondary to a mass effect or to underlying disease. We analyzed pre- and postnatal characteristics of 26 patients with severe CDH and 23 patients with severe CPAM from 2009 to 2012. Severe mediastinal compression, indicated by a low cardiothoracic ratio, was evident in both groups. However, fetuses with severe CDH had smaller pulmonary arteries bilaterally and higher pulsatility indices in the ipsilateral lung than those with severe CPAM. Prenatal modified McGoon indices were significantly lower in patients with CDH versus CPAM. Consistent with these prenatal measurements, postnatal PH was seen more frequently in patients with CDH compared to CPAM. Patients with severe CDH have prenatal evidence of pulmonary vascular remodeling compared to patients with severe CPAM. These results suggest a multifactorial origin for PH in CDH and support the idea of using prenatal medical therapies to promote vascular remodeling in these patients.