Type I and II interferons (IFNs) exert opposing effects on the progression of multiple sclerosis, even though both IFNs use the signal transducer and activator of transcription 1 (STAT1) as a signaling mediator. Here we report that STAT1-deficient mice expressing a transgenic T cell receptor against myelin basic protein spontaneously develop experimental autoimmune encephalomyelitis with dramatically increased frequency. The heightened susceptibility to this autoimmune disease appears to be triggered by a reduced number as well as a functional impairment of the CD4+ CD25+ regulatory T cells in STAT1-deficient animals. Adoptive transfer of wild-type regulatory T cells into STAT1-deficient hosts is sufficient to prevent the development of autoimmune disease. These results demonstrate an essential role of STAT1 in the maintenance of immunological self-tolerance.