IDDM (type I diabetes) is generally believed to result from T-cell-mediated autoimmune destruction of the insulin-producing β-cells in the pancreatic islets of Langerhans. In the last few years, considerable progress has been made with regard to the identification and characterization of candidate autoantigens recognized by autoantibodies; several of these candidate autoantigens are recognized by T-cells, including insulin, GAD65 and GAD67, heat-shock protein 65 (hsp65), and islet-cell antigen 69 (ICA69). In addition to these, a number of unidentified β-cell antigens, including insulin-secretory granule membrane proteins and a 38-kDa protein, have been shown to stimulate T-cells of IDDM patients. However, T-cell autoreactivity to islet antigens is not specific for IDDM, and the T-cell target antigens are not specific for β-cells. Moreover, the autoantigens involved in the initiation of the insulitis must be defined, and the mechanism of the T-cell-dependent β-cell destruction remains to be unraveled. This review focuses on T-cell autoreactivity in IDDM in humans and the implications of the present knowledge for immunointervention and monitoring of immunotherapeutic trials.