Although this paper does not deal specifically with arthritis, its relevance lies in the intriguing speculations it raises on the common basis for autoimmunity and tumor immunity. It suggests that a regulatory T cell controls both tumor specific and self-reactive T cells. Studies show that a subpopulation of CD4+ T cells prevents autoimmune disease in a number of rodent models. Many studies have previously suggested this regulatory T cell population is CD25+ CD4+, and is anergic (non-proliferative) in normal mice. Stimulation of the regulatory T cells suppresses the activation/proliferation of other CD4+ or CD8+ T cells in an antigen non-specific manner. Removal of CD25+ CD4+ T cells elicits autoimmunity, and enhances immune responses to non-self antigens such as allogeneic tissue grafts. Here the authors investigate the hypothesis that removal of CD25+ CD4+ immunoregulatory T cells may evoke immune responses to autologous tumor cells by allowing activation of anti-tumor effector cells. To investigate the role of immunoregulatory CD25+ CD4+T cells in tumor immunity.