Presented is the case of alveolar cell carcinoma in a 66-year-old woman that manifested diffuse multiple cystic cavity-forming lesions with infiltrations into the lungs. An autopsy revealed a bronchiolo-alveolar cell adenocarcinoma with mediastinal metastatic lymph nodes. Most of the cavities, whose walls were microscopically lined by tall columnar tumor cells in the pattern typical of an alveolar cell carcinoma, were round-shaped and had thin walls that showed no necrosis and were filled with mucus that contained tumor cells. The majority of these cavities were characteristic tension cavities or cysts resulting from an increased in the intraalveolar pressure caused by the mucus produced by the tumor cells in the alveolar space.