A 71-year-old man presented with a thin-walled cavity in his left lung in November 2006. A previous chest CT in 2003 showed a small thin-walled cavity in his left lingula. Although no obvious change was observed in 2004, the cavity increased its size from 11 mm to 14 mm in diameter and the wall became thicker in June 2006. On the first visit to our hospital in November 2006, the diameter of the cavity was 30 mm and some part of the wall was thinner than on the previous CT. The patient developed pneumothorax one month later and underwent segmentectomy of the left lingula after unsuccessful thoracic drainage. Poorly differentiated adenocarcinoma was identified in both the pleura and the inner wall around the cavity. Lung adenocarcinoma with gradual enlargement of a thin-walled cavity causing pneumothorax has never been reported before. We report here the natural course of lung adenocarcinoma with a thin-walled cavity.