Anomalies of naturally occurring CD4+ regulatory T cells (Treg) cause severe autoimmune/inflammatory diseases in humans and rodents. The transcription factor Foxp3 is currently the most specific marker for natural CD4+ Treg, but it would be useful if other Treg markers, particularly cell surface molecules, could be elucidated. We demonstrate in this study that the vast majority of Foxp3-expressing CD4+ T cells (whether CD25+ or CD25−) show constitutive high-level expression of glucocorticoid-induced TNFR family-related gene/protein (GITR). Transfer of T cell or thymocyte suspensions depleted of GITR high cells produces in BALB/c nude mice a wider spectrum and more severe forms of autoimmune diseases than does transfer of similar cell suspensions depleted of CD25+ CD4+ T cells only. Notably, mice that receive cells depleted of GITR high populations develop severe multiorgan inflammation that includes fatal autoimmune myocarditis resembling giant cell myocarditis in humans, accompanying high-titer anti-myosin autoantibodies. Similar transfer of GITR high-depleted cells from prediabetic NOD mice to NOD-SCID mice accelerates the development of diabetes and induces skeletal muscle myositis and other autoimmune/inflammatory diseases. We conclude that GITR high, Foxp3-expressing natural Treg, containing both CD25+ and CD25− cell populations, contribute to preventing a variety of autoimmune/inflammatory diseases, and depletion of these cells allows the activation of even weak or rare autoreactive T cells yielding widespread severe autoimmune disease. Diseases induced in this way include many which have been suspected of an autoimmune etiology in humans without much evidence. GITR high, Foxp3-expressing natural Treg represent a potential target for the treatment and prevention of these diseases.