We experienced a case of postoperative mental change with seizure after propofol-N₂O anesthesia for thoracoscopic sympathectomy. A 23-year-old male patient was anesthetized with N₂O-O₂-propofol, and postoperative pain control was achieved with intravenous morphine. Bilateral electrocauterization of the upper dorsal sympathetic chain at the T2 and T3 level was performed with thoracoscopy. Thoracoscopy was done with carbon dioxide insufflation. In the postoperative recovery room, the patient experienced seizure, fever and a mental change. He was transferred to the intensive care unit and treated with acyclovir, mannitol, diphenylhydantoin and dexamethasone. Fourteen days after the operation, the patient was discharged with a clear mentality.