We report a case of lung adenocarcinoma in which cystic lesions enlarged and multiloculated over 2 years. Histological examination of the resected specimen found proliferation of nonmucinous adenocarcinoma cells along the alveolar walls, revealing bronchioloalveolar cell carcinoma type extension. In cystic lesions, particularly those not associated with inflammation, lung adenocarcinoma, particularly bronchioloalveolar cell carcinoma type, should be a diagnostic consideration. .