Infants born with congenital diaphragmatic hernia (CDH) die because their lungs are hypoplastic. In the fetal lamb, the lung made hypoplastic by compression with an intrathoracic balloon can, if decompressed in utero, grow and develop enough to permit survival at term. To study the efficacy and feasibility of in utero repair of CDH, we created diaphragmatic hernias in fetal lambs at approximately 100 days gestation and corrected them surgically at approximately 120 days. Repair of the diaphragm with closure of the abdomen resulted in postoperative fetal death in six lambs. Acute studies demonstrated that increased intraabdominal pressure compromises blood flow in the umbilical vein and produces severe fetal distress. When a silastic patch was used to enlarge the abdomen after reduction of the viscera and repair of the diaphragm, six of nine lambs were viable after term delivery. In sacrificed lambs, the lungs were well expanded, mature histologically, and greatly increased in size. Correction of CDH in utero appears physiologically sound and technically feasible.