A population of CD25+ CD4+ regulatory T cells (T regs) functions to maintain immunological self tolerance by inhibiting autoreactive T cell responses. CD25+ CD4+ T regs are present in low, but steady, numbers in the peripheral lymphoid tissues of healthy mice. Recent studies have shown that IL-2 is an essential growth factor for these cells. How this cytokine functions to regulate CD25+ CD4+ T reg homeostasis and prevent autoimmune disease remains unknown. In conventional CD4+ T cells, IL-2 triggers signaling pathways that promote proliferation and survival by activating the STAT5 transcription factor and by increasing the expression of the antiapoptotic protein, Bcl-2. We show here that bcl-2 deficiency does not affect CD25+ CD4+ T reg homeostasis, and that ectopic expression of this molecule fails to rescue CD25+ CD4+ T reg numbers or to prevent the development of autoimmunity in IL-2-deficient mice. Furthermore, transient activation of STAT5 is sufficient to increase CD25+ CD4+ T reg numbers in IL-2-deficient mice. Our study uncovers an essential role for STAT5 in maintaining CD25+ CD4+ T reg homeostasis and self-tolerance.