In T cell-deficient conditions, naïve T cells undergo spontaneous “homeostatic” proliferation in response to contact with self-MHC/peptide ligands. With the aid of anin vitrosystem, we show here that homeostatic proliferation is also cytokine-dependent. The cytokines IL-4, IL-7, and IL-15 enhanced homeostatic proliferation of naïve T cellsin vitro. Of these cytokines, only IL-7 was found to be critical; thus, naïve T cells underwent homeostatic proliferation in IL-4−and IL-15−hosts but proliferated minimally in IL-7−hosts. In addition to homeostatic proliferation, the prolonged survival of naïve T cells requires IL-7. Thus, naïve T cells disappeared gradually over a 1-month period upon adoptive transfer into IL-7−hosts. These findings indicate that naïve T cells depend on IL-7 for survival and homeostatic proliferation.