Lethal autoimmunity associated with IL-2Rβ-deficient mice is prevented after thymic transgenic expression of wild-type IL-2Rβ in IL-2Rβ^−/− mice (Tg −/− mice). Here, we show that CD4⁺CD25⁺ regulatory T cells were not readily detected in IL-2Rβ^−/− mice, but the production of functional CD4⁺CD25⁺ T cells was reconstituted in Tg −/− mice. Adoptive transfer of normal CD4⁺CD25⁺ T cells into neonatal IL-2Rβ-deficient mice prevented this lethal autoimmune syndrome. The CD4⁺CD25⁺ T cells in disease-free adult IL-2Rβ-deficient recipient mice were present at a near normal frequency, were solely donor-derived, and depended on IL-2 for expansion. These observations indicate that the essential function of the IL-2/IL-2R system primarily lies at the level of the production of CD4⁺CD25⁺ regulatory T cells.