We report on a patient presenting with a bronchioloalveolar carcinoma fortuitously detected in the wall of a bronchogenic cyst. Evidence suggests that unstable epithelial cells contained within the cyst wall may lead to premalignant proliferation and neoplasia. In the current case, we demonstrated an increased proliferative activity in some areas of the cyst consistent with atypical adenomatous hyperplasia. Hence, we stress the importance of close follow-up of all suspected congenital lung cysts because of their potential malignant degeneration.