The phenotype of NFATc2^−/− c3^−/− (double knockout [DKO]) mice implies a disturbed regulation of T cell responses, evidenced by massive lymphadenopathy, splenomegaly, and autoaggressive phenomena. The population of CD4⁺ CD25⁺ T cells from DKO mice lacks regulatory capacity, except a small subpopulation that highly expresses glucocorticoid-induced tumor necrosis factor receptor family–related gene (GITR) and CD25. However, neither wild-type nor DKO CD4⁺ CD25⁺ regulatory T cells (T reg cells) are able to suppress proliferation of DKO CD4⁺ CD25⁻ T helper cells. Therefore, combined NFATc2/c3 deficiency is compatible with the development of CD4⁺ CD25⁺ T reg cells but renders conventional CD4⁺ T cells unresponsive to suppression, underlining the importance of NFAT proteins for sustaining T cell homeostasis.