OBJECTIVE: Wilms' tumor gene (WT1), located on chromosome 11, encodes a transcription factor that contributes to the carcinogenesis of uterine sarcomas. To expand the knowledge on the biological role of WT1 in other uterine cancers, we focused on its detection in endometrial carcinoma. METHODS: In total, 36 paraffin-embedded tumors were available for WT1 immunohistochemical (IHC) analysis including endometrial endometrioid carcinoma (n=24), serous carcinoma (n=9) and clear cell carcinoma (n=3). Three slides from different sites of the tumor were analysed. Of these tumors, 32 snap frozen tissue samples were available for RT-PCR (endometrioid carcinoma (23), serous carcinoma (7) and clear cell carcinoma (2)). To compare, WT1 expression was also evaluated by IHC in benign endometrium (12) and benign endometrial polyps (5). RESULTS: WT1 positivity was noticed in tumor cells and endothelial cells, lining the intratumoral blood vessels. Overall, 72% (26/36) of tumors stained positive for WT1. RT-PCR results showed WT1 positivity in 75% (24/32) of samples. Comparing the staining patterns of the 3 different bioptic sites, tumor heterogeneity was demonstrated in the majority (72%) of samples. CONCLUSION: Although WT1 is expressed in a majority of endometrial carcinomas, a heterogeneous staining pattern is observed. This information is important for WT1-directed immunotherapy.