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.