The size of the T lymphocyte pool is maintained by regulation of T cell production, proliferation, and survival. Under the pressure of a T lymphopenic environment, mature naive T cells begin to proliferate in the absence of Ag, a process called homeostatic expansion. Homeostatic expansion involves TCR recognition of self peptide/MHC ligands, but less is known about the soluble factors that regulate this process. Here we show that IL-12 dramatically enhanced the homeostatic proliferation of CD8 T cells. In contrast, IL-2 had no beneficial effect on homeostatic expansion and, in fact, inhibited T cell expansion induced by IL-12. Using gene-targeted mice, we showed that IL-12 acted directly on the T cells to enhance homeostatic expansion, but that IL-12 cannot override the requirement for TCR interaction with self peptide/MHC ligands in homeostatic expansion. These data indicate that inflammatory cytokines may modulate T cell homeostasis after lymphopenia and have implications for regulation of the T cell repertoire and autoimmunity.