Activation and expansion of T and B lymphocytes and myeloid cells are controlled by Foxp3+ regulatory T cells (T reg cells), and their deficiency results in a fatal lympho- and myeloproliferative syndrome. A role for T reg cells in the homeostasis of innate lymphocyte lineages remained unknown. Here, we report that T reg cells restrained the expansion of immature CD127+ NK cells, which had the unique ability to up-regulate the IL2Rα (CD25) in response to the proinflammatory cytokine IL-12. In addition, we observed the preferential accumulation of CD127+ NK cells in mice bearing progressing tumors or suffering from chronic viral infection. CD127+ NK cells expanded in an IL-2–dependent manner upon T reg cell depletion and were able to give rise to mature NK cells, indicating that the latter can develop through a CD25+ intermediate stage. Thus, T reg cells restrain the IL-2–dependent CD4+ T cell help for CD127+ immature NK cells. These findings highlight the adaptive control of innate lymphocyte homeostasis.