A healthy 34-year-old man had a mediastinal cyst on the imaging study. Surgical treatment was performed. The cyst was diagnosed as a thoracic duct cyst from its anatomic location and contents. Pathologic examination found it to be consistent with thoracic duct cyst. Endothelial cells on its luminal surface were identified by an immunohistologic stain with the factor VIII-related antigen. Twenty-six cases of thoracic duct cysts have been reported. We report an additional case and review the previously reported cases. We found that the ligation of the inferior pedicle of the cyst is essential to prevent postoperative chylothorax.