Transcription factors of the nuclear factor of activated T cells (NFAT) family are thought to regulate the expression of a variety of inducible genes such as interleukin-2 (IL-2), IL-4, and tumor necrosis factor-α. However, it remains unresolved whether NFAT proteins play a role in regulating transcription of the interferon- γ (IFN-γ) gene. Here it is shown that the transcription factor NFAT1 (NFATc2) is a major regulator of IFN-γ production in vivo. Compared with T cells expressing NFAT1, T cells lacking NFAT1 display a substantial IL-4–independent defect in expression of IFN-γ mRNA and protein. Reduced IFN-γ production by NFAT1^−/−× IL-4^−/− T cells is observed after primary in vitro stimulation of naive CD4⁺ T cells, is conserved through at least 2 rounds of T-helper cell differentiation, and occurs by a cell-intrinsic mechanism that does not depend on overexpression of the Th2-specific factors GATA-3 and c-Maf. Concomitantly, NFAT1^−/−× IL-4^−/− mice show increased susceptibility to infection with the intracellular parasiteLeishmania major. Moreover, IFN-γ production in a murine T-cell clone is sensitive to the selective peptide inhibitor of NFAT, VIVIT. These results suggest that IFN-γ production by T cells is regulated by NFAT1, most likely at the level of gene transcription.