CD4⁺CD25⁺ regulatory T (T_reg) cells have a crucial role in maintaining immune tolerance. Mice and humans born lacking T_reg cells develop severe autoimmune disease^,, and depletion of T_reg cells in lymphopenic mice induces autoimmunity^,. Interleukin (IL)-2 signaling is required for thymic development, peripheral expansion and suppressive activity of T_reg cells. Animals lacking IL-2 die of autoimmunity^,, which is prevented by administration of IL-2–responsive T_reg cells. In light of the emerging evidence that one of the primary physiologic roles of IL-2 is to generate and maintain T_reg cells, the question arises as to the effects of IL-2 therapy on them. We monitored T_reg cells during immune reconstitution in individuals with cancer who did or did not receive IL-2 therapy. CD4⁺CD25^hi cells underwent homeostatic peripheral expansion during immune reconstitution, and in lymphopenic individuals receiving IL-2, the T_reg cell compartment was markedly increased. Mouse studies showed that IL-2 therapy induced expansion of existent T_reg cells in normal hosts, and IL-2–induced T_reg cell expansion was further augmented by lymphopenia. On a per-cell basis, T_reg cells generated by IL-2 therapy expressed similar levels of FOXP3 and had similar potency for suppression compared to T_reg cells present in normal hosts. These studies suggest that IL-2 and lymphopenia are primary modulators of CD4⁺CD25⁺ T_reg cell homeostasis.