Transgenic mice broadly expressing JunD (Ubi‐junD^m) appear phenotypically normal, but have strongly reduced numbers of peripheral lymphocytes. JunD overexpression in lymphocytes does not protect from numerous apoptotic insults; however, transgenic T cells proliferate poorly and exhibit impaired activation due to reduced levels of IL‐4, CD25 and CD69. Consistently, in the absence of JunD (junD^−/−) T cells hyperproliferate following mitogen induction. Moreover, transgenic T helper (Th) 2 cells have decreased IL‐4 and IL‐10 expression, whereas junD^−/− Th2 cells secrete higher amounts of both Th2 cytokines. Th1‐polarized junD^−/− CD4⁺ T cells display enhanced IFN‐γ cytokine production associated with upregulated T‐bet expression and downregulated expression of suppressor of cytokine signaling‐1. These novel findings demonstrate a regulatory role of JunD in T lymphocyte proliferation and Th cell differentiation.