Ten cases of congenital cystic adenomatoid malformation of the lung are presented, together with a review of the literature. Two infants were stillborn; the others died shortly after birth. Eight had hydrops, and in eight the pregnancy was complicated by hydramnios. Grossly, one or two lobes of the lung were affected, the remaining lobe(s) and the opposite lung being hypoplastic. The lesions were solid, or cystic. Apart from a solitary cutaneous hamartoma, there was no associated anomaly. Microscopically, two patterns were seen--solid and mixed. "Mucigenic epithelium" was seen in six cases. The pathogenesis of this lesion, together with that of the hydrops and hydramnios, is discussed. It is concluded that congenital cystic adenomatoid malformation of the lung is a distinct pathological entity; its prompt recognition and surgical correction should result in normal survival.