Interleukin-2 (IL-2) is a well-known T-cell growth factor, which is traditionally implicated in the agonistic stimulation of immune responses. Recent work, however, has uncovered an unexpected function of this cytokine. In particular, IL-2 appears crucial to maintaining peripheral tolerance by supporting the survival and function of CD25+ CD4+ regulatory T cells. A recent study examining the role of IL-2 in the peripheral generation of regulatory T cells from apparently naïve T cells has gone some way to reconciling the seemingly opposing functions of this cytokine.