Abstract Dendritic cell (DC)–based immunotherapy can trigger effective immune responses against cancer in human patients. Although accompanied by little toxicity, further improvements are needed to optimize immune responses for fully satisfactory clinical outcomes. IFNγ, a potent inducer of T helper type 1 immune responses, is considered an important tool to realize improvements. In this study, we sought to clarify the effect of IFNγ on the maturation and activation of DCs and the clinical outcome of DC-based cancer therapy in dogs. In vitro experiments indicated that IFNγ significantly enhanced the expression of immune stimulatory molecules and interleukin-12 by DCs derived from canine monocytes. IFNγ also significantly strengthened DC-mediated growth suppression against tumor cell lines. DC inoculation with concomitant delivery of IFNγ into primary or recurrent tumors elicited significant clinical responses, including four complete responses and two partial responses against malignant tumors, also eliciting partial responses against benign but actively growing tumors. Together, our results indicate that combining IFNγ and DCs could induce strong immune responses against tumors, significantly improving clinical outcomes. The present study of dogs bearing common types of cancer in humans offers a unique line of support for the development of human cancer therapies. Cancer Res; 70(18); 7093–101. ©2010 AACR.