We investigated the usefulness of measuring the serum CA19-9 concentration in the diagnosis and therapeutic monitoring of endometriosis, and compared it with the serum CA125 concentration. We added an immunohistochemical study on CA19-9 and CA125 to find their localization in the various lesions of endometriosis. 1. The positive rate of serum CA19-9 in patients with endometriosis is 52.0% and that of CA125 is 48.0%. When both markers are examined, their positive rate increases to 71.4%. 2. As the Beecham classification stage advanced, the serum CA19-9 positive rate tended to increase. 3. The patients with endometriosis were followed up after surgery or during Danazol therapy. CA19-9 correlated closely with the clinical course. 4. The immunohistochemistry of CA19-9 shows that it localizes in chocolate cysts, but not in the lesions of adenomyosis. CA125 is seen in both lesions. The localizing of CA19-9 is thus seen to be different from that of CA125. It is concluded that the measurement of the serum CA19-9 concentration combined with that of CA125 is useful in the diagnosis and therapeutic monitoring of endometriosis.