The transcriptional repressor Gfi1 is a nuclear zinc-finger protein that is expressed in T cell precursors in the thymus, but is down-regulated in mature, resting T cells. Gfi1 expression rises transiently to levels seen in thymocytes upon antigenic activation. We show here that lack of Gfi1 causes delayed cell cycle entry and apoptosis after antigenic stimulation in both mature CD4+ and CD8+ T cells ex vivo. DNA micro-array analysis demonstrated that this correlated with an up-regulation of the death receptor CD95, the proapoptotic factors Bad and Apaf1 and the cell cycle inhibitor p21, and a down-regulation of Bcl-2 expression in Gfi1-/- T cells. Surprisingly, while Gfi1-deficient CD4+ T cells showed the same defective behavior in vivo, Gfi1-deficient CD8+ T cells showed no aberration in vivo and were fully able to mount an anti-viral immune response. This indicates that Gfi1 exerts different functions in CD4+ and CD8+ T cells very likely by maintaining different genetic programs in both cell types, and appears to be essential for the CD4 helper T cell immune response but dispensable for the function of cytotoxic CD8+ T cells.