Abstract
Patients with congenital lung cysts are at increased risk of developing carcinoma, but the mechanisms concerned are not clear. The case of a young adult who developed a bronchioloalveolar carcinoma associated with a cystic congenital adenomatoid malformation is reported. The adjacent lung showed an unusual intra-alveolar hyperplasia of mucous cells. Two further cases of congenital adenomatoid malformation are also described; both patients presented in infancy and showed similar mucous cell hyperplasia in alveoli surrounding the cysts. In all three cases the staining characteristic of the mucus was identical with that of normal bronchial mucous glands. It is suggested that the benign proliferation represents a premalignant lesion. Its presence in infants shows that it may occur at an early age and reinforces the need for early removal of congenital lung cysts.