Mice lacking the transcription factor Foxp3 (Foxp3⁻) lack regulatory T (T_reg) cells and develop fatal autoimmune pathology. In Foxp3⁻ mice, many activated effector T cells express self-reactive T cell receptors that are expressed in T_reg cells in wild-type mice. Thus, in wild-type mice, most self-reactive thymocytes escaping negative selection are diverted into the T_reg lineage, and whether T_reg cells are critical in self-tolerance in wild-type mice remains unknown. Here, acute in vivo ablation of T_reg cells demonstrated a vital function for T_reg cells in neonatal and adult mice. We suggest that self-reactive T cells are continuously suppressed by T_reg cells and that when suppression is relieved, self-reactive T cells become activated and facilitate accelerated maturation of dendritic cells.