The telomerase reverse transcriptase, TERT, is an attractive target for human cancer vaccination because its expression is reactivated in a conspicuous fraction of human tumors. Genetic vaccination with murine telomerase (mTERT) could break immune tolerance in different mouse strains and resulted in the induction of both CD4⁺ and CD8⁺ telomerase-specific T cells. The mTERT-derived immunodominant epitopes recognized by CD8⁺ T cells were further defined in these mouse strains and used to track immune responses. Antitumor efficacy of telomerase-based vaccination was investigated in two cancer models closely resembling human diseases: the TRAMP transgenic mice for prostate cancer and a carcinogen-induced model for colon cancer. TERT overexpression in tumor lesions was shown in both models by immunohistochemistry, thus reinforcing the similarity of these tumors to their human counterparts. Repeated immunizations with mTERT-encoding DNA resulted in a significant delay of tumor formation and progression in both the prostate cancer and the colon cancer models. Moreover, evaluation of the intratumoral infiltrate revealed the presence of telomerase-specific T cells in vaccinated mice. The safety of vaccination was confirmed by the absence of histomorphologic changes on postnecropsy analysis of several organs and lack of adverse effects on blood cell counts. These results indicate that TERT vaccination can elicit antigen-specific immunosurveillance and imply this antigen as a potential candidate for preventive cancer vaccines. [Cancer Res 2008;68(23):9865–74]