OX40 ligand (OX40L) expressed on APCs, and its receptor, OX40 present on activated T cells, are members of the TNF/TNFR family, respectively, and have been located at the sites of inflammatory conditions. We have observed in OX40L-deficient mice (OX40L−/−) an impaired APC capacity and in our recently constructed transgenic mice expressing OX40L (OX40L-Tg), a markedly enhanced T cell response to protein Ags. Using these mice, we demonstrate here the critical involvement of the OX40L-OX40 interaction during the T cell priming events in the occurrence of experimental autoimmune encephalomyelitis (EAE). In OX40L−/− mice, abortive T cell priming greatly reduced the clinical manifestations of actively induced EAE, coupled with a reduction in IFN-γ, IL-2, and IL-6 production in vitro. Adoptive transfer experiments however revealed an efficient transfer of disease to OX40L−/− mice using wild-type donor T cells, indicating an intact capacity of OX40L−/− mice to initiate effector responses. On the other hand, OX40L−/− donor T cells failed to transfer disease to wild-type recipient mice. Furthermore, OX40L-Tg mice developed a greater severity of EAE despite a delayed onset, while both OX40L-Tg/CD28−/− and OX40L-Tg/CD40−/− mice failed to develop EAE demonstrating a requisite for these molecules. These findings indicate a pivotal role played by OX40L in the pathogenesis of EAE.