Islet transplantation is one of the most promising therapies for type 1 diabetes (T1D). A major issue in islet transplantation is the loss of graft function at late phase. Several studies suggested the involvement of islet-specific T-cells in such islet graft dysfunction. In this study, we investigated the breadth and type of glutamic acid decarboxylase 65 (GAD65)-specific T-cells in T1D patients after allogeneic islet transplantation. Peripheral blood mononuclear cells (PBMCs) were obtained from islet-transplanted T1D patients during insulin-independent period and cultured for 7 days with pools of GAD65 overlapping peptides in the presence of IL-2. Cytokine secretion profiles of peptide-reactive T-cells were analyzed after a short-term restimulation with the same peptides by a multiplex bead-based cytokine assay and by an intracytoplasmic cytokine detection assay. Robust GAD65-specific CD4(+) and CD8(+) T-cell responses were detected in patients who eventually developed chronic graft dysfunction. Multiple GAD65 peptides were found to induce specific T-cell responses in these patients, indicating that the repertoire of GAD65-specific T-cells was broad. Furthermore, GAD65-specific CD4(+) T-cells were composed of heterogeneous populations, which differentially expressed cytokines including IFN-γ and type 2 cytokines, but not IL-10. In contrast, patients who showed only marginal GAD65-specific T-cell responses maintained substantially longer graft survival and insulin independence. In conclusion, our study suggests that the emergence of islet-specific T-cells precedes the development of chronic graft dysfunction in islet-transplanted patients. Thus, our observations support the hypothesis that these islet-specific T-cells contribute to the development of chronic islet graft dysfunction.