Purpose: Ovarian clear cell adenocarcinoma (OCCA) has been reported to display different characteristics from other histological types of epithelial ovarian cancer, and especially differs from serous adenocarcinoma. We investigated plasminogen activator inhibitor-1 (PAI-1) expression in patients with OCCA and attempted to assess its biological significance. Methods: Fifty-seven patients with OCCA were enrolled. We used formalin-fixed, paraffin-embedded sections of the primary tumor obtained at the first operation to investigate the immunohistochemical expression of PAI-1 and the association of PAI-1 expression with various clinicopathologic factors. Results: The 57 patients were classified into a high PAI-1 expression group and a low expression group. Comparison between the two groups revealed that the percentage of patients with concomitant endometriosis was significantly larger in the high expression group, while the percentage of Stage I patients with positive peritoneal cytology was significantly larger in the low expression group. Comparison of cumulative 5-year survival rates showed that the high expression group had a better prognosis than the low expression group. Conclusion: These data suggest an association between concomitant endometriosis and increased expression of PAI-1 in OCCA. The data also suggest that PAI-1 expression influences both peritoneal dissemination of early OCCA and the prognosis.