The patient was a 59-year-old man with a 5-month history of cough, difficulty breathing, and palpitations while working. Initial examination at the internal medicine department of our hospital in August 1992 revealed a mass in the left side of the neck, and he was admitted for further examination and treatment. Preoperative investigations revealed a cystic lesion extending from the left neck area to the mediastinum. The mass was resected on August 13, 1992. Based on the intraoperative findings and postoperative histological examination, a diagnosis of thoracic duct cyst was made. The postoperative course was good and the patient was discharged after two weeks. This is extremely rare case, and the Trap door surgical approach proved useful for resection.