Cell surface carbohydrate expression strongly influences the biological characteristics of cancer cells. Especially, it is known that the change of sialic acid expression could be related to the invasive and metastatic potentials of tumors. This study aimed to investigate sialidase expression of ovarian cancer cells and to evaluate the relationship between plasma membrane-associated sialidase (NEU3) expression and various clinicopathological factors in ovarian clear cell adenocarcinoma patients. In 18 cell lines derived from human ovarian cancers (including clear cell, mucinous, and serous adenocarcinoma), sialidase mRNA expression was evaluated by RT-PCR. NEU1 and NEU3 expression levels were found to be elevated in most cell lines while NEU2 and NEU4 expression was rarely elevated. Interestingly, NEU3 expression was detected in all clear cell adenocarcinoma cell lines. In 71 patients with ovarian clear cell adenocarcinoma, treated at Keio University Hospital from February 1983 to February 2002, NEU3 expression was examined by immunohistochemical staining of surgical specimens and clinicopathological factors were reviewed. NEU3 expression was found to be positive in 77.5% of all cases. Furthermore, a high level of NEU3 expression was significantly correlated with T3 factor of pTNM classification on univariate and multivariate analysis. This is the first report to show that NEU3 is expressed in most of ovarian clear cell adenocarcinoma. And our results show that NEU3 expression is correlated with T factor (pTNM classification) in ovarian clear cell adenocarcinoma.