Adoptive transfer of T cells expressing chimeric NKG2D (chNKG2D) receptors, a fusion of NKG2D and CD3zeta, can lead to long-term, tumor-free survival in a murine model of ovarian cancer. To determine the mechanisms of chNKG2D T cell antitumor efficacy, we analyzed how chNKG2D T cells altered the tumor microenvironment, including the tumor-infiltrating leukocyte populations. chNKG2D T cell treatment of mice bearing ID8 tumor cells increased the number and activation of NK cells and increased the activation of host CD8+ T cells within the tumor. Foxp3+ regulatory T cells at the tumor site decreased more than 300-fold after chNKG2D T cell treatment. Tumor-associated regulatory T cells expressed cell surface NKG2D ligands and were killed by chNKG2D T cells in a perforin-dependent manner. chNKG2D T cells also altered the function of myeloid cells at the tumor site, changing these cells from being immunosuppressive to enhancing T cell responses. Cells isolated from the tumor produced elevated amounts of IFN-gamma, NO, and other proinflammatory cytokines after chNKG2D T cell treatment. ChNKG2D T cells required perforin, IFN-gamma, and GM-CSF to induce a full response at the tumor site. In addition, transfer of chNKG2D T cells into mice bearing tumors that were established for 5 weeks led to long-term survival of the mice. Thus, chNKG2D T cells altered the ovarian tumor microenvironment to eliminate immunosuppressive cells and induce infiltration and activation of antitumor immune cells and production of inflammatory cytokines. This induction of an immune response likely contributes to chNKG2D T cells' ability to eliminate established tumors.