ENDOSCOPIC SYMPATHECTOMY IS a new trend for the treatment of hyperhidrosis palmaris. It is a simple and effective technique; however, it carries some recognized risks such as Horner's syndrome and pneumohemothorax. We recently encountered a case complicated by the development of a chylothorax. The patient was a 23-year-old healthy woman with profuse palmar sweating. She developed an intractable dry cough after a transthoracic endoscopic sympathectomy. A chest x-ray revealed a left pleural effusion. A chylous effusion was found after thoracentesis and fluid analysis. The pleural effusion resolved after chest tube drainage and diet control. Although endoscopic sympathectomy is a simple and quick procedure, unusual complications, such as chylothorax, may occur. Appropriate early recognition and treatment can prevent a disastrous result.