In the homeostasis of the immune system regulatory cells play a major role. Removal of one group of regulatory cells, the CD25(+)CD4(+) T cells, leads to autoimmune manifestations in experimental animal models, and reintroduction of this population prevents disease. This study addresses the role of such regulatory T cells in humans with an autoimmune disease, where we demonstrate the presence of CD25(bright)CD4(+) T cells in the target organ of patients with active rheumatoid arthritis. The patients displayed an enrichment of CD25(bright)CD4(+) T cells in synovial fluid as compared to peripheral blood. These cells are functional regulatory cells, as they were able to suppress in vitro proliferation of autologous T cells, both from synovial and peripheral blood origin. Although the frequency of CD25(bright)CD4(+) T cells varied between patients, it was found to be constant over time in any one joint during each relapse. Numbers were also comparable in two inflamed knee joints of one and the same patient, emphasizing the symmetry of the disease. In summary, it is striking that in addition to all activated, potentially pathological T cells the synovial fluid from RA patients also contains CD25-expressing CD4(+) T cells with a regulatory capacity.