Regulatory T cells (Treg cells), which are lymphocyte subsets capable of suppressing immune responses, appear to play a crucial role in maintaining immune homeostasis and mediating peripheral tolerance. However, Treg cells also accumulate in cancer patients and have been implicated in tumor immune escape. The forkhead box P3 (FOXP3) transcription factor is currently regarded as the most specific and reliable marker for Treg cells in men. We investigated the frequency and characterized the distribution of FOXP3(+) cells in renal cell carcinoma (RCC) patients, focusing on the tumor microenvironment. FOXP3 expression was assessed in kidney tissue samples from 32 RCC patients by reverse transcriptase polymerase chain reaction (RT-PCR) and immunohistochemistry. Both conventional and quantitative RT-PCR disclosed higher FOXP3 expression levels in RCC than in adjacent normal renal tissue. Immunohistochemical staining of FOXP3-expressing cells confirmed the accumulation of FOXP3(+) cells in tumor tissue, particularly at the border between malignant and adjacent benign kidney tissues. Our findings indicate that Treg cells accumulate at the tumor invasion zone and could thus be part of an immune escape mechanism of RCC that promotes disease progression.