T-cell–based immunotherapies can be effective in the treatment of large vascularized tumors, but they rely on adoptive transfer of substantial numbers (∼20 million) of tumor-specific T cells administered together with vaccination and high-dose interleukin (IL)-2. In this study, we report that ∼10,000 T cells gene-engineered to express a single-chain IL-12 molecule can be therapeutically effective against established tumors in the absence of exogenous IL-2 and vaccine. Although IL-12–engineered cells did not perist long-term in hosts, they exhibited enhanced functionality and were detected in higher numbers intratumorally along with increased numbers of endogenous natural killer and CD8⁺ T cells just before regression. Importantly, transferred T cells isolated from tumors stably overproduced supraphysiologic amounts of IL-12, and the therapeutic effect of IL-12 produced within the tumor microenvironment could not be mimicked with high doses of exogenously provided IL-12. Furthermore, antitumor effects could be recapitulated by engineering wild-type open-repertoire splenocytes to express both the single-chain IL-12 and a recombinant tumor-specific T-cell receptor (TCR), but only when individual cells expressed both the TCR and IL-12, indicating that arrested migration of T cells at the tumor site was required for their activities. Successful tumor eradication was dependent on a lymphodepleting preconditioning regimen that reduced the number of intratumoral CD4⁺ Foxp3⁺ T regulatory cells. Our findings reveal an approach to genetically modify T cells to reduce the cell number needed, eliminate the need for vaccines or systemic IL-2, and improve immunotherapy efficacy based on adoptive transfer of gene-engineered T cells. Cancer Res; 70(17); 6725–34. ©2010 AACR.