Lung volume and lung mechanics were measured in 19 infants, aged 2 to 8 weeks, with congenital pulmonary hypoplasia of various causes. These included nine infants with congenital diaphragmatic hernia, five infants with Werdnig-Hoffman disease of intrauterine onset, and five infants with isolated pulmonary hypoplasia or associated anomalies such as thoracic dystrophy and spina bifida. Infants were studied within the first two months of life in order to exclude the effects of natural adaptation and adverse influences such as lower respiratory tract infection. Lung volume and airway resistance were estimated by whole body plethysmography, total pulmonary resistance, and dynamic lung compliance with esophageal balloons and pneumotachography. Reductions in dynamic lung compliance were found in seven of the nine infants with congenital diaphragmatic hernia, probably reflecting lung overdistention after operative repair. Low airflow resistance was found in 12 infants, representing subjects from each category. The low airflow resistance may be related to reductions in airway generation number found when the disturbance of lung development occurs before 16 weeks of gestational age. This finding suggests a role for measurements of airflow resistance in characterizing the type of pulmonary hypoplasia.