OBJECTIVES: Gemcitabine (GEM) is a pyrimidine nucleoside analogue that is a new chemotherapeutic agent used for treating various cancers. Because accumulating evidence indicates that GEM may activate host immune responses, its potential as an immune modulator in cancer chemotherapy has generated considerable interest. MATERIALS AND METHODS: In the present study, we investigated the antitumor effects of GEM using a mouse oral cancer model using immunological analyses. We examined apoptotic cell death of tumor cells with GEM treatment both in vitro and in vivo. We also investigated whether in vivo administration of GEM affected the distributions of immune cells, tumor-cell surface expression levels of immune accessory molecules and T cell immune responses in tumor-bearing mice. RESULTS: GEM induced significant oral cancer-cell apoptosis in vitro, and in vivo GEM administration markedly attenuated established mouse tumor growth. In vivo GEM administration decreased the numbers of both myeloid-derived suppressor cells (MDSCs) and B cells in tumor-bearing mice and enhanced dendritic cell maturation. Moreover, GEM treatment upregulated tumor-cell surface expressions of several immune accessory molecules and adhesion molecules, including CD80, CD86, CD40, ICAM-1, VCAM-1, and P-selectin. Remarkably, these tumor cells augmented tumor specific T-cell responses. CONCLUSION: These results suggest that GEM can induce host antitumor immune responses, which would facilitate antitumor effects in the treatment of oral cancer.