Abstract Purpose: To predict the potential antitumor effect of antigen-specific T cells in melanoma patients, we investigated T-cell effector function in relation to tumor-escape mechanisms. Experimental Design: CD8+ T cells isolated from tumor, adjacent normal skin, and peripheral blood of 17 HLA-A2+ patients with advanced-stage melanoma were analyzed for their antigen specificity and effector function against melanocyte differentiation antigens MART-1, gp100, and tyrosinase by using HLA-A2/peptide tetramers and functional assays. In addition, the presence of tumor-escape mechanisms PD-L1/PD-1 pathway, FoxP3 and loss of HLA or melanocyte differentiation antigens, both required for tumor cell recognition and killing, were studied. Results: Higher percentages of melanocyte antigen-specific CD8+ T cells were found in the melanoma tissues as compared with adjacent normal skin and peripheral blood. Functional analysis revealed 2 important findings: (i) in 5 of 17 patients, we found cytokine production after specific peptide stimulation by tumor-infiltrating lymphocytes (TIL), not by autologous peripheral blood lymphocytes (PBL); (ii) CD8+ T cells from 7 of 17 patients did not produce cytokines after specific stimulation, which corresponded with significant loss of tumor HLA-A2 expression. The presence of other tumor-escape mechanisms did not correlate to T-cell function. Conclusions: Our data show that functional T-cell responses could be missed when only PBL and not TIL are evaluated, emphasizing the importance of TIL analysis for immunomonitoring. Furthermore, loss of tumor HLA-A2 may explain the lack of T-cell functionality. These findings have important implications for selecting melanoma patients who may benefit from immunotherapy. Clin Cancer Res; 17(17); 5736–47. ©2011 AACR.