To determine whether preoperative stabilization and delay of operative repair of congenital diaphragmatic hernia (CDH) may decrease operative risk, we performed serial pulmonary function tests on 22 newborn infants with CDH and on four infants without pulmonary hypoplasia (two with lleal atresia and two with tracheoesophageal anomalies) who served as control subjects. We used 2 passive respiratory mechanics technique to measure respiratory system compliance. All patients with CDH had respiratory distress immediately after birth, and required mechanical ventilation. Thirteen babies underwent emergency repair (six survived, seven died); nine of them received extracorporeal membrane oxygenation (ECMO) after the operation (two survived, seven died). Operative repair was delayed deliberately for 2 to 11 days in nine infants with severe hypoxemia. Six immediately received ECMO for 4 to 10 days; one died of intraventricular hemorrhage, and five survived and later underwent surgical repair. The seventh patient did not receive ECMO but appeared to have respiratory distress syndrome of infancy and improved after administration of synthetic surfactant. Improvement was seen in two additional infants who received conventional assisted ventilation during a 48-hour delay before surgery, and survived. In all, eight of nine infants who underwent preoperative stabilization survived (p<0.05 compared with survival after emergency surgery). Following surgical repair immediately after birth, respiratory system compliance improved only slightly during the first week of life, a time when control infants had a rapid increase in respiratory system compliance (p<0.001). In contrast, respiratory system compliance increased nearly twofold in the nine patients undergoing preoperative stabilization (p<0.02). Preoperative ECMO was associated with an increase in respiratory system compliance of more than 60% for 1 week, a significant difference from respiratory system compliance among patients undergoing emergency CDH repair (p<0.05). These observations provide physiologic evidence of possible benefits of preoperative stabilization before repair of CDH.