The frequent overexpression of prostate-derived Ets factor (PDEF) mRNA in ovarian cancer has been previously reported. The aim of this study was to evaluate PDEF protein expression in ovarian cancer and how this expression might vary at different stages of epithelial ovarian tumors in comparison to normal ovary. A new rabbit polyclonal antibody to PDEF was prepared, and immunohistochemistry was performed on tissue sections from 12 normal ovaries, 10 cases of benign serous cystadenoma, 17 cases of low malignant potential tumor, 19 cases of stage 1, and 15 cases of advanced stage primary epithelial (serous) ovarian carcinomas and their peritoneal metastases. Expression levels were assessed based on the percentage of positively staining cells and the intensity of staining. All 12 normal ovary and 10 benign serous cystadenoma cases were negative for PDEF expression. In contrast, 6 of 17 (35%) low malignant potential tumors, 5 of 19 (27%) stage 1, and 5 of 15 (33%) advanced stage ovarian tumors stained positive for PDEF expression. Together, these results show frequent overexpression of PDEF protein in epithelial ovarian tumors and its lack of expression in normal ovary and cystadenomas, and this supports a role for PDEF in ovarian tumorigenesis. Furthermore, these results suggest that PDEF is a potential marker and target in ovarian cancer.