Objective.To evaluate (a) the expression of CD31 in benign, borderline, and maligant ovarian tumors; (b) the correlation between CD31 expression and the clinicopathological parameters; and (c) the diagnostic interest of serological soluble CD31 (sCD31) in patients with ovarian tumors. Methods.The intratumoral microvessel density was evaluated by an immunohistochemical technique with the monoclonal antibody JC70 against CD31 at two dilutions in 20 benign, 20 borderline, and 20 malignant tumors of the ovary. Serological determinations of sCD31 with ELISA technique was performed in 35 patients with ovarian tumors (24 benign, 5 borderline, and 6 malignant tumors). Results.The expression of CD31 was higher in ovarian carcinomas than in borderline and benign tumors (P< 0.001) irrespective of the dilutions of the antibody used. In ovarian carcinomas, a correlation was observed between CD31 expression and the stage of the disease, the histologic type, the degree of histological differentiation, and the survival of the patients. In borderline tumors, no correlation was noted between CD31 expression and the clinicopathologic parameters. No difference in serological levels of sCD31 was noted according to histologic types. Conclusion.CD31 immunostaining may have a prognostic relevance in ovarian carcinoma but seems to be of limited value in borderline tumors. Serological levels of sCD31 have no diagnostic interest in ovarian tumors.