Immunotherapy based on adoptive transfer of tumor antigen-specific CD8þ T cell (TC) is generally limited by poor in vivo expansion and tumor infiltration. In this study, we report that activated STAT5 transcription factors (STAT5CA) confer high efficiency on CD8þ effector T cells (eTC) for host colonization after adoptive transfer. Engineered expression of STAT5CA in antigen-experienced TCs with poor replicative potential was also sufficient to convert them into long-lived antigen-responsive eTCs. In transplanted mastocytomaor melanoma-bearing hosts, STAT5CA greatly enhanced the ability of eTCs to accumulate in tumors, become activated by tumor antigens, and to express the cytolytic factor granzyme B. Taken together, these properties contributed to an increase in tumor regression by STAT5CA-transduced, as compared with untransduced, TCs including when the latter control cells were combined with infusion of interleukin (IL)-2/anti–IL-2 complexes. In tumors arising in the autochthonous TiRP transgenic model of melanoma associated with systemic chronic inflammation, endogenous CD8þ TCs were nonfunctional. In this setting, adoptive transfer of STAT5CA-transduced TCs produced superior antitumor effects compared with nontransduced TCs. Our findings imply that STAT5CA expression can render TCs resistant to the immunosuppressive environment of melanoma tumors, enhancing their ability to home to tumors and to maintain high granzyme B expression, as well as their capacity to stimulate granzyme B expression in endogenous TCs. Cancer Res; 72(1); 76–87. 2011 AACR.