Congenital adenomatoid malformation (CAM) of the lung was diagnosed at 30 weeks gestation. The mother presented with preterm labor and polyhydramnios. A complex cystic mass was seen in the right lung of the fetus. Additional radiographic and sonographic investigations prior to delivery allowed differentiation of this rare lesion from other cystic thoracic pathology of the fetal period. Careful hospital obstetric management of the mother and fetus for over 3 weeks allowed the delivery of an infant with adequate pulmonary maturity to permit stabilization and surgery on the baby in the first days of life.