GATA-3, a C2C2-type zinc finger transcription factor, regulates many steps of T cell development and differentiation. It is also required for optimal production of type 2 cytokines by CD8+ T cells. However, its role in the development and function of this subset of T cells is still poorly characterized. In this paper, we report that GATA-3 is required for MHC-mediated positive selection and final maturation of CD8 single-positive thymocytes. Deficiency of GATA-3 mediated by a CD4cre transgene led to age-dependent lymphadenopathy partly because of abnormal expansion of CD8+ T cells driven by a cell-extrinsic mechanism. Paradoxically, GATA-3–deficient CD8+ T cells were hyporesponsive to Ag stimulation due to a defect in the maintenance/progression, but not initiation, of activation signals. More importantly, GATA-3–deficient CD8+ T cells were less efficient in killing Ag-bearing tumor cells in vivo. Taken together, our data further expand the role of GATA-3 in T cells.