Ovarian carcinoma has been difficult to diagnose at an early stage. Recently, it has been recognized that the measurement of blood N-ERC/mesothelin levels aids early detection in and postoperative therapeutic monitoring of patients with mesothelioma, who have been exposed to asbestos. ERC/mesothelin has also been reported to be expressed in ovarian carcinoma. We determined serum N-ERC/mesothelin levels in patients with ovarian carcinoma using an enzyme-linked immunosorbent assay (ELISA). In addition, we immunohistochemically evaluated surgically resected specimens for C-ERC/mesothelin expression. As a result, of the 32 patients with ovarian tumors (18 carcinoma, 2 borderline tumors), one patient with serous adenocarcinoma showed increased N-ERC/ mesothelin levels. Immunohistochemically, of the 20 ovarian tumor (carcinoma and borderline tumor) specimens evaluated for serum N-ERC/mesothelin, 9 (45.0%) were positive for C-ERC/mesothelin. The C-ERC/mesothelin-positive specimens were found to be serous and clear cell adenocarcinomas. If serum N-ERC/mesothelin, which is considered useful for early detection in and therapeutic monitoring of patients with mesothelioma, may also be used for ovarian carcinoma monitoring, it may be a valuable serum tumor marker for the early detection of ovarian carcinoma.