With the recent advent of prenatal ultrasound as a routine screening procedure, diagnosis of congenital cystic lung disease has been made in utero, raising the possibility of elective surgery for these lesions early in infancy before the patient develops respiratory distress or potentially life-threatening infection. From 1979 to 1989 six cases of congenital lung cyst were diagnosed in utero by prenatal ultrasound and followed during pregnancy. Two of the six were not confirmed after birth because the mothers preferred an abortion. The remaining four cases were studied periodically during gestation by ultrasonography. At birth, the first infant developed respiratory distress and underwent urgent left upper lobectomy and left lower segmentectomy at age 18 hours. The other three underwent elective lobectomy at age 10 days, 3 months, and 7 months, respectively. The fourth infant had a normal chest x-ray and ultrasound at birth, and the congenital cysts were confirmed by computed tomography scan. The pathological diagnosis in all four cases was cystic adenomatoid malformation. In two cases, intraoperative measurement of pulmonary function demonstrated significant improvement after resection of the affected lobe. We conclude that congenital lung cysts can be accurately diagnosed by prenatal ultrasound "screening" as early as 18 to 24 weeks' gestation. Advantages of early diagnosis include the option of moving the mother and unborn child to a high-risk obstetrical center for urgent operation on the newborn infant if necessary. Otherwise, once the diagnosis is confirmed, surgical correction can be performed electively and safely before respiratory distress or pulmonary infection complicates the infant's growth and development.