Glucocorticoid-induced tumour necrosis factor receptor-related receptor (GITR) costimulates functions of both effector and regulatory T cells. The administration of agonistic anti-GITR monoclonal antibodies efficiently enhances various T-cell-mediated immune responses; however, it is unknown to what extent the ligand of GITR (GITRL) contributes to T-cell responses. We investigated the involvement of endogenously expressed GITRL on dendritic cells and ectopically expressed GITRL on tumours in T-cell-mediated immunity. Expression of GITRL on dendritic cells in secondary lymphoid organs was limited, and treatment with anti-GITRL monoclonal antibodies did not substantially affect T-cell-mediated immunity to alloantigens, a specific protein antigen (ovalbumin), or tumour antigens. The introduction of GITRL promoted anti-tumour immunity in four tumour models. Tumour-associated GITRL greatly augmented the effector function of CD8(+) T cells and enhanced the contribution of CD8(+) T cells. These events reduced the crucial contribution of CD25(+) CD4(+) regulatory T cells, which were found to inhibit immunity against tumours lacking GITRL. Peritumoral injection of GITRL tumour vaccine efficiently inhibited the growth of established tumours. Our results suggest that the ectopic expression of GITRL in tumour cells enhances anti-tumour immunity at peripheral tumour sites. Consequently, the combined use of a GITRL tumour vaccine with methods aimed at enhancing the activation of host antigen-presenting cells in secondary lymphoid tissues may be a promising strategy for tumour immunotherapy.