Besides positive and negative selection of T cells, another function of the thymus in maintaining immunological self‐tolerance is the production of CD25⁺CD4⁺ regulatory T cells capable of preventing autoimmune disease. They acquire the regulatory activity through the thymic selection process, and are released to the periphery as a functionally and phenotypically mature population. Our recent study with transgenic mice in which every class II MHC molecule covalently binds the same single peptide has revealed that a particular self‐peptide/MHC ligand in the thymus can positively select a broad repertoire of functionally mature CD25⁺CD4⁺ regulatory T cells as well as naïve T cells. Interestingly, the regulatory T cells bear higher reactivity than other T cells to the selecting ligand in the thymus even after negative selection by the ligand. This broad repertoire and high self‐reactivity of CD25⁺CD4⁺ regulatory T cells, together with their high level expression of various accessory molecules, may guarantee their prompt and efficient activation upon encounter with a diverse range of self peptide/MHC complexes in the periphery, ensuring dominant control of self‐reactive T cells.