Prenatal ultrasonography makes possible the diagnosis of congenital cystic lung disease in utero, enabling elective surgery for the lesions early in infancy before development of respiratory distress or potentially life-threatening infection. From 1986 to 1994, ten infants with congenital lung cyst underwent corrective surgery. Nine of them had a prenatal diagnosis of congenital lung cyst confirmed by ultrasound or computed tomography (CT) after birth. In 7 of these infants, congenital adenomatoid malformation (CAM) was found at surgery, and pulmonary sequestration was found and removed in two other infants. Two of the nine infants had respiratory distress at birth; the seven others were asymptomatic when surgery was performed (between 2 and 6 months of age). In 4 out of 9 newborn a plain chest radiography at birth was normal and a CT scan only showed the cysts prenatally observed at ultrasound. Intraoperative measurement of pulmonary function in the asymptomatic infants demonstrated significant improvement after resection of the affected lobe. One other infant had normal lung fields at the time of prenatal diagnosis. However, a severe acute pneumonia with abscess of the lobe became life-threatening at 4.5 months. CT showed a large congenital lung cyst, treated initially with antibiotics and extirpated at age 6 months with great difficulty. Pulmonary function did not improve postoperatively. Examinations on follow-up (8 months-8 years; 5 of 10 infants longer than 4 years) have shown that all 10 of the infants are healthy and have normal pulmonary function. Pneumonia in the controlateral lung was found in 3 children (3-6 years after the operation), one of them had respiratory distress at birth and another had the life-threatening pneumonia preoperatively. Thus, early surgery is indicated when prenatal diagnosis of congenital cystic disease has been confirmed after birth.