We report the case of a 70-year-old man (ASA physical status 2) who developed massive pneumocephalus caused by a fistula between the subarachnoid and pleural spaces following a left pneumonectomy. After an uneventful immediate postoperative period, the patient was readmitted to the recovery care unit with dyspnea, intense headache, confusion, and diminished level of consciousness. Computed tomography confirmed a cerebrospinal fluid fistula secondary to the opening of the intradural space during tumor resection. Treatment was conservative, consisting of rest in a slightly Trendelenburg position, antibiotic prophylaxis to prevent meningitis, and a water seal on the thoracic drainage tube.