Cytokine signals are known to contribute to CD8+ memory T cell homeostasis, but an exact understanding of the mechanism (s) has remained elusive. We have now investigated the role of Stat5 proteins in this process. Whereas Stat5a and Stat5b KO mice have decreased numbers of CD8+ T cells, Stat5-transgenic mice have an increased number of these cells. Stat5b-transgenic mice exhibit increased Ag-induced cell death of CD4+ T cells and augmented proliferation and Bcl-2 expression in CD8+ T cells, providing a basis for this finding. Moreover, CD8+ memory T cells are substantially affected by Stat5 levels. These findings identify Stat5 proteins as critical signaling mediators used by cytokines to regulate CD8+ T cell homeostasis.