I investigated 250 patients with mature cystic teratoma (MCT) of ovary, using an enzyme immunoassay to determine the tissue source of their high serum levels of CA19-9. The mean preoperative serum CA19-9 level was 79.4 U/ml, and the mean postoperative level was far lower (27.4 U/ml). There were 41 patients positive for CA19-9. In the 31 with serum CA19-9 levels over 101 U/ml, CA19-9 was immunohistochemically demonstrated in the bronchial glands (14 cases) and bronchial mucosa (8 cases) of the MCTs. Six cases of bronchial glands and bronchial mucosa showed positive for CA19-9 by immunohistochemical stainings. CA19-9 levels were higher in the cyst fluid of MCTs than in the serum. These findings suggested two routes for the transfer of tumor-produced CA19-9 into the bloodstream: 1) direct secretion from MCT bronchial tissue, and 2) leakage from MCT cysts into the surrounding capillaries through the interstitium.