Multiple TLR agonists have been shown to have antitumor effects in animal models. However, the therapeutic efficacy of TLR agonist monotherapy in cancer treatment has been limited, and the mechanisms of failure remain unknown. We demonstrate that topical treatment with a TLR-7 agonist, imiquimod, can elicit significant regression of spontaneous breast cancers in neu transgenic mice, a model of human HER-2/neu+ breast cancer. However, tumor growth progressed once imiquimod therapy was ended. Gene expression analysis using tumor-derived RNA demonstrated that imiquimod induced high levels of IL-10 in addition to TNF-α and IFN-γ. Elevated levels of circulating IL-10 were also detected in sera from imiquimod-treated mice. Elevated serum IL-10 appeared to be derived from IL-10 and dual cytokine secreting (IFN-γ+ and IL-10+) CD4+ T cells rather than CD4+ CD25+ Foxp3+ T regulatory cells, which were also induced by imiquimod treatment. Blockade of IL-10, but not TGF-β, enhanced the antitumor effect of imiquimod by significantly prolonging survival in treated mice. These data suggest that the excessive inflammation induced by TLR agonists may result in a self-regulatory immunosuppression via IL-10 induction and that blocking IL-10 could enhance the therapeutic efficacy of these agents.