Cystic lymphangiomas are extremely rare mediastinal tumors in adult patients. They are usually located in the neck (75%) or axillary region (20%), with only 1% localized in the mediastinum. When present in the mediastinum, they can be located in all mediastinal compartments, with a predilection for the anterior mediastinum. The tumor is usually noticed when compression symptoms occur, or when it is accidentally found on the chest X-ray. Complete surgical resection is indicated for this tumor. We present a case and discuss the occurrence, pathogenesis, and management of the postoperative chylothorax of a cystic lymphangioma.