We have previously observed that treatment of both human and murine dendritic cells (DCs) treated with ultra-low doses of the antitumor agent 5-fluorouracil (5-FU) improves their maturation/activation and increases the antigen presentation activity. Thus, the present study was designed to evaluate whether the modulatory effects of low concentration of 5-FU combined with DC-based vaccine also occurs in vivo using the experimental model of murine colorectal cancer MC-38 cells. Tumor-bearing C57BL/6 mice were subcutaneously inoculated with syngeneic MC-38 cells and ten days later we started to treat them twice a week with a DC vaccine plus 5-FU. Thirty days after tumor implantation, the tumors were evaluated for the infiltration of MDSC and T cell subsets. Combination of DC vaccine with a low dose of 5-FU induced tumor regression in 77% of the animals, while progressed in all other control groups. Combinatory therapy also reduced the number of circulating MDSC in comparison with non-treated animals. Our results indicate that low dose 5-FU enhances the therapeutic efficacy of DC vaccine.