For unknown reasons, advanced age remains a dominant predictor of poor clinical outcome for nearly all cancers. A decrease in the production of T cells by the thymus accompanies normal aging and parallels the age-dependent increase in cancer progression, but the specific impact of immunity on tumor progression in general is unknown. Glioblastoma multiforme (GBM), the most common primary brain neoplasm, is characterized by rapid age-dependent rates of progression and death. In this study, we show levels of CD8(+) recent thymic emigrants (RTEs) accounted for the prognostic power of age on clinical outcome in GBM patients. CD8(+) RTEs, typically a tiny proportion of CD8(+) T cells, remarkably accounted for the majority of tumor Ag-binding small precursor cells in PBMC from these patients and from healthy individuals. Large blasting tumor Ag-binding cells comprised of CD8(+) RTEs and phenotypically related cells were predominantly expanded following experimental vaccination of GBM patients. Quantification of CD8(+) RTE expansion in vivo correlated strongly with vaccine-elicited cytokine responses, and estimated numbers of expanding CD8(+) RTEs were consistent predictors of clinical outcome in vaccinated GBM patients. Targeted mutant (CD8beta(-/-)) mice specifically deficient in thymic CD8(+) T cell production uniquely displayed an age-specific decrease in glioma host survival as well as a strong correlation between host survival and thymus cellular production. These findings suggest that levels and function of newly produced CD8(+) T cells critically influence age-dependent cancer mortality and exert one of the strongest known influences on GBM outcome by predominantly mediating clinically beneficial antitumor immune responses.