Epithelial ovarian cancer (EOC) is a heterogeneous disease. It has been shown that the expression of Wilms tumor gene (WT1) protein was different among the various EOC subtypes. The aims of this study were to determine the expression of WT1 protein in EOCs and to compare WT1 protein expression with EOC subtype, histological grade, and the International Federation of Gynecology and Obstetrics (FIGO) stage. This retrospective study was conducted in 102 patients with EOCs who underwent debulking surgery. The standard histological sections were used to assess pathologic parameters. Expression of WT1 protein in the tumor specimens was immunohistochemically analyzed by using monoclonal anti-WT1 antibody. The mean age of EOC patients was 61.2 +/- 12.5 years (range 23-74 years). The most EOC patients had serous carcinomas (76,47%), histological grade 3 tumors (70,59%) and FIGO stage III disease (78,43%). Expression of WT1 protein was found in 80,39% of EOC specimens. The predominant expression pattern was homogenous, which was observed in 66,67% of WT1 positive EOC specimens. Expression of WT1 protein was significantly associated with serous subtype (p<0.001), higher histological grade (p<0.001) and advanced EOC stage (p<0.001). In conclusion, results of this study suggest that the expression of WT1 protein may be a marker of biological aggressiveness of EOCs.