Despite expression of the high-affinity IL-2R, CD4+ CD25+ regulatory T cells (Tregs) are hypoproliferative upon IL-2R stimulation in vitro. However the mechanisms by which CD4+ CD25+ T cells respond to IL-2 signals are undefined. In this report, we examine the cellular and molecular responses of CD4+ CD25+ Tregs to IL-2. IL-2R stimulation results in a G 1 cell cycle arrest, cellular enlargement and increased cellular survival of CD4+ CD25+ T cells. We find a distinct pattern of IL-2R signaling in which the Janus kinase/STAT pathway remains intact, whereas IL-2 does not activate downstream targets of phosphatidylinositol 3-kinase. Negative regulation of phosphatidylinositol 3-kinase signaling and IL-2-mediated proliferation of CD4+ CD25+ T cells is inversely associated with expression of the phosphatase and tensin homologue deleted on chromosome 10, PTEN.