cAMP response element binding protein (CREB) is a critical regulator of diverse stimulus-dependent transcriptional events. Following TCR stimulation, CREB is rapidly induced in CD4+ Th cell precursors, but not in effector Th cells. However, its role in mature T cell function is incompletely defined. Transgenic mice expressing a CREB dominant-negative (dn) mutation in the T cell lineage exhibited normal T cell development in the thymus, normal T cell homeostasis in the periphery, and normal T cell clonal expansion following Ag challenge. However, this mutation caused selective inhibition of Th cell function in vitro and in vivo, and increased susceptibility of Th cells to activation-induced cell death. Th cells expressing the CREB-dn mutation contained reduced levels of the inhibitor of programmed cell death, BCL-2; overexpression of BCL-2 in transgenic mice reversed both susceptibility to activation-induced cell death in CREB-dn T cells and the defect in effector cytokine production. Thus, CREB plays a critical role in Th cell function and development of Th cell-mediated adaptive immune responses, at least in part, by inhibiting stimulus-dependent cell death.