We experienced 12 cases of congenital cystic adenomatoid malformation of the lung (CCAM) including 6 cases diagnosed antenatally. They were classified into three groups according to the clinical manifestations. Group A was associated with hydrops fetalis (n = 3), group B presented with respiratory distress symptoms after birth (n = 6), and group C showed no respiratory symptoms (n = 3). All cases of group A were lost because of hydrops and respiratory failure due to pulmonary hypoplasia. Because a compression of the mass is thought to be a cause of hydrops, this group is considered to be a good indication for fetal treatment. All cases of group B showed progressive respiratory symptoms a few days after birth which were successfully treated surgically. In 2 of 3 cases of group C, the lesions decreased in size both antenatally and postnatally. We conclude that serial sonographic evaluations for fetal CCAM are important. If the fetus develops hydrops, fetal surgery is to be considered. If not, however, fetal surgery should not be done, because some lesions can shrink in size, or even disappear, while others can be treated successfully after birth by lobectomy or even segmentectomy.