Naturally occurring CD4+ CD25+ regulatory T (nTreg) cells are essential for maintaining T cell tolerance to self Ags. We show that discrimination of human Treg from effector CD4+ CD25+ non-nTreg cells and their selective survival and proliferation can now be achieved using rapamycin (sirolimus). Human purified CD4+ CD25 high T cell subsets stimulated via TCR and CD28 or by IL-2 survived and expanded up to 40-fold in the presence of 1 nM rapamycin, while CD4+ CD25 low or CD4+ CD25− T cells did not. The expanding pure populations of CD4+ CD25 high T cells were resistant to rapamycin-accelerated apoptosis. In contrast, proliferation of CD4+ CD25− T cells was blocked by rapamycin, which induced their apoptosis. The rapamycin-expanded CD4+ CD25 high T cell populations retained a broad TCR repertoire and, like CD4+ CD25+ T cells freshly obtained from the peripheral circulation, constitutively expressed CD25, Foxp3, CD62L, glucocorticoid-induced TNFR family related protein, CTLA-4, and CCR-7. The rapamycin-expanded T cells suppressed proliferation and effector functions of allogeneic or autologous CD4+ and CD8+ T cells in vitro. They equally suppressed Ag-specific and nonspecific responses. Our studies have defined ex vivo conditions for robust expansion of pure populations of human nTreg cells with potent suppressive activity. It is expected that the availability of this otherwise rare T cell subset for further studies will help define the molecular basis of Treg-mediated suppression in humans.