Immunotherapies aimed at enhancing natural or endogenous antitumor T-cell immunity in patients affected by advanced malignancies are currently being implemented in the clinic with promising results. In order to optimize therapeutic protocols and monitor the effectiveness of such therapies, reliable biomarkers are needed. We used CD11a, an integrin that is upregulated on the surface of effector and memory CD8⁺ T cells, and PD-1, an immunoregulatory receptor expressed by activated T cells, as biomarkers to identify, quantify and monitor endogenous tumor-reactive cytotoxic T lymphocytes (CTLs) in two mouse tumor models and in the peripheral blood of 12 patients affected by Stage IV melanoma. High expression levels of CD11a and PD-1 were detected among CD8⁺ T cells residing within primary and metastatic murine tumor sites, as well as in spontaneous murine breast cancer tissues. In the peripheral blood of melanoma patients, tumor antigen-specific CD8⁺ T cells were associated with a population of CD11a^high CD8⁺ T cells that co-expressed high levels of PD-1. Healthy donors exhibited a comparatively much lower frequency of such PD-1⁺CD11a^highCD8⁺ T cells. Phenotypic analyses demonstrated that CD11a^highCD8⁺ T cells are proliferating (Ki67⁺) and activated (CD62L^-CD69⁺). Increased CD11a^highCD8⁺ T cells and delayed tumor growth were observed in PD-1 deficient mice, suggesting that the antitumor effector functions of CD8⁺ T cells is compromised by an elevated expression of PD-1. The CD11a^highCD8⁺ T-cell population expresses high levels of PD-1 and presumably constitutes the cellular target of PD-1 blockade therapy. The expression level of CD11a and PD-1 by CD8⁺ T cells may therefore represent a novel biomarker to identify and monitor endogenous tumor-reactive CTLs. This may not only provide an immunological readout for evaluating the efficacy of immunotherapy but also contribute to the selection of cancer patients who are likely to benefit from anti-PD-1 therapy.