Abstract Adoptive immunotherapy with CD8+ T cells has achieved some success in treating established cancer. Response rates are increased by preconditioning the patient with lymphodepleting whole body irradiation (WBI) prior to T cell transfer, including an increased fraction of complete responses. Using a mouse model of autochthonous brain cancer, we previously demonstrated that adoptive transfer of tumor-specific T cells following WBI promotes complete regression of established tumors and long-term survival. However, the mechanisms required to achieve complete responses remain to be defined. Here we investigate kinetic requirements for adoptive T cell transfer to achieve tumor control following WBI. To test the hypothesis that adoptive transfer early after irradiation is required for maximum long-term tumor control, we delayed adoptive transfer of T cells to tumor-bearing mice at varying intervals following irradiation. We show that delay of adoptive transfer did not affect initial T cell priming or the timing of initial appearance of activated T cells at the tumor site. However, we observed a modest decrease in T cell accumulation at the earliest appearance of cells in the brain (5 days post-transfer). Tumors were completely eliminated within 10 days in mice that received T cell transfer one day following WBI, while delayed transfer abrogated this effect. When adoptive transfer was delayed by one week following WBI, incomplete tumor regression was observed at 10 days post-transfer. Although mice in this group lived significantly longer than control animals, they eventually succumbed to lethal tumor recurrence. Our data suggest that lymphodepleting doses of WBI promote a prolonged window for effective T cell priming against the endogenous tumor antigen, but that optimal therapeutic potential requires T cell transfer early after irradiation conditioning. Supported by research grant CA02500 from the National Cancer Institute/National Institutes of Health. Citation Format: Eugene M. Cozza, Lynn R. Budgeon, Neil D. Christensen, Todd D. Schell. Regression of established tumors requires timely adoptive T cell transfer following whole body irradiation. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3975. doi:10.1158/1538-7445.AM2013-3975