Abstract Renal cell carcinoma is frequently infiltrated by cells of the immune system. This makes it important to understand interactions between cancer cells and immune cells so they can be manipulated to bring clinical benefit. Here, we analyze subsets and functions of T lymphocytes infiltrating renal cell tumors directly ex vivo following mechanical disaggregation and without any culture step. Subpopulations of memory and effector CD4+ Th1, Th2, and Th17 and CD8+ Tc1 cells were identified based on surface phenotype, activation potential, and multicytokine production. Compared with the same patient's peripheral blood, T lymphocytes present inside tumors were found to be enriched in functional CD4+ cells of the Th1 lineage and in effector memory CD8+ cells. Additionally, several populations of CD4+ and CD8+ regulatory T cells were identified that may synergize to locally dampen antitumor T-cell responses. [Cancer Res 2009;69(21):8412–9]