A 20-year-old female was diagnosed as mediastinal tumor and admitted to our hospital for the operation. Among the preoperative examinations, the serum level of CA 19-9 was within normal limits. The fluid inside the cystic tumor which was removed during the operation showed high levels of CA 19-9. We discussed the relationship between CA 19-9 and cystic lesions. Histopathological study revealed that CA 19-9 was produced in only epithelial cell of the cystic lesion. And we speculated that examination of serum CA 19-9 helps the diagnosis of cystic lesions.