Lymphangioma, or cystic hygroma, a malformation resulting from the sequestration of lymph vessels, is owing to a failure of the primitive jugular lymphatic sacs to join with the venous system. They are rare benign lesions found more frequently in children than in adults. We experienced a case of huge mediastinal lymphangioma in 37-year-old female patient. It was associated with a cervical prolongation, and caused deviation and compression of the trachea and the heart. The cervicomediastinal lymphangioma was completely resected by means of median sternotomy, and the post-operative course was unevenful.