CD8+ T cells are critical in human type 1 diabetes and in the NOD mouse. In this study, we elucidated the natural history of isletspecific glucose-6-phosphatase catalytic subunit-related protein (IGRP)-specific CD8 + T cells in NOD diabetes using MHCtetramer technology. IGRP 206-214-specific T cells in the peripheral lymphoid tissue increased with age, and their numbers correlated with insulitis progression. IGRP 206-214-specific T cells in the peripheral lymphoid tissue expressed markers of chronic Ag stimulation, and their numbers were stable after diagnosis of diabetes, consistent with their memory phenotype. IGRP206-214- specific T cells in NOD mice expand, acquire the phenotype of effector-memory T cells in the islets, and emigrate to the peripheral lymphoid tissue. Our observations suggest that enumeration of effector-memory T cells of multiple autoantigen specificities in the periphery of type 1 diabetic subjects could be a reliable reporter for progression of islet pathology.