Hypoxia occurs in many tumors and reduces the effectiveness of radio- and chemotherapy. Hypoxia also impedes immune responses to tumors, reducing T lymphocyte production of cytokines such as interleukin-2 (IL-2) and interferon gamma, as well as the survival and proliferation of these cells. We constructed a lentiviral vector encoding a bidirectional hypoxia-inducible responsive element (HRE) derived from human vascular endothelial growth factor, which drives the hIL-2 gene and a marker gene. We used a model of human B cell lymphoma to show that tumor-specific T cells modified with this vector upregulate hIL-2 expression when oxygen tension is low in vitro and in vivo. The consequence of this effect is to increase T-cell survival and proliferation whilst sustaining effector function, even in O₂ concentrations as low as 1%. The phenotype of the transduced cells is unchanged, as is their ability to migrate to tumor. HRE-IL-2-modified cytotoxic T lymphocytes (CTLs) produce faster and more complete tumor regression than parental CTLs and increase overall survival. Hypoxia-resistant T cells may thus be of value in the treatment of human tumors in which areas of hypoxia may otherwise account for resistance to this therapeutic strategy.