We report 2 cases of lung cancer incidentally detected following pneumothorax. Case 1:A 40-year-old man complaining of dyspnea was admitted with right pneumothorax. Chest computed tomography (CT) after chest drainage showed a cavitary nodule with pleural indentations in the right lower lobe. It was indicated at surgery that pneumothorax was caused by perforation of the tumor into the pleural cavity. Right lower lobectomy was performed because the pathological diagnosis of the nodule was a large cell carcinoma. The final histopathological diagnosis was stage II A (pT2aN1M0). The patient died of recurrence 14 months after surgery. Case 2:A 47-year-old man who admitted with right pneumothorax was found to have a nodule with pleural indentations closely a bulla at the apex of the right lung by chest CT after chest drainage. Pneumothorax was indicated to be caused by rupture of the bulla at surgery. Right upper lobectomy was performed because the pathological diagnosis of the nodule was a squamous carcinoma. The final histopathological diagnosis was stage I A (pT1bN0M0). The patient is alive at 2 years after the operation without recurrence. Lung cancer detected following pneumothorax which was caused by perforation of the tumor is generally considered to have poor prognosis. Whereas, prognosis of lung cancer incidentally detected following pneumothorax depends on its staging.