Histological slides of 789 cases of ovarian tumor registered by the Japanese Gynecologic Oncology Group, sponsored by the Ministry of Health and Welfare, Japan, were reviewed by three certified pathologists in order to evaluate the observer disagreement among the pathologists and the reproducibility of the histological classification. In 53.0% of the cases, all observers agreed upon the diagnosis. At least two of them agreed in 88.4% of the cases. The observer disagreement was highest for low potential malignancy (borderline malignancy) tumors (44.4%), including serous, mucinous, and endometrioid tumors. The results of the study clearly indicate the necessity of establishing a slide review system in order to obtain reliable data for comparative research. Moreover, the histological criteria, especially of tumors of borderline malignancy, should be more clearly defined to obtain high reproducibility of ovarian tumor diagnosis.