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Background: Regulatory T cells (Treg) are subsets of lymphocytes able to suppress immune responses and appear crucial in maintaining immune homeostasis by mediating peripheral tolerance. Increased Treg frequencies, however, have also been found in cancer patients implicating their involvement in tumor immune escape. Currently FOXP3 (transcription factor forkhead box P3) is accepted to be the most specific and reliable marker for regulatory T cells in men. We investigated the frequency of FOXP3+ cells in patients with renal cell cancer (RCC), focusing on the tumor microenvironment, and characterized their distribution. Material and Methods: Kidney tissue samples from 32 patients with RCC were characterized concerning FOXP3 expression applying reverse-transcriptase polymerase chain reaction (RT-PCR) as well as immunohistochemistry. Results: Using both conventional and quantitative RT-PCR to detect and quantify FOXP3 expression we were able to demonstrate a higher FOXP3 expression in RCC compared to adjacent corresponding normal renal tissue. Immunohistochemical investigation of FOXP3 expressing cells could not only confirm that there were higher numbers of FOXP3+ cells in tumor tissue but also revealed that these cells accumulated particularly in the frontier area between malignant and benign adjacent kidney tissue. Conclusion: Our findings indicate that Treg might accumulate at the tumor invasion and progression zone and thus could be part of an immunological escape mechanism for renal cell carcinoma.