T-cell-mediated loss of pancreatic beta-cells is the crucial event in the development of type 1 diabetes. The phenotypic characteristics of disease-associated T-cells in type 1 diabetes have not yet been defined. The negative results from two intervention trials (the Diabetes Prevention Trial-Type 1 Diabetes and the European Nicotinamide Diabetes Intervention Trial) illustrate the need for technologies to specifically monitor ongoing autoimmune reactions. We used fluorescence-activated cell sorter analysis to study surface marker expression on T-cell lines specific for two major type 1 diabetes autoantigens, GAD65 and proinsulin. We then applied this knowledge in a cross-sectional approach to delineate the phenotype of circulating memory T-cells. The autoreactive T-cells of patients could be distinguished from those of control subjects by their coexpression of CD25 and CD134. Autoantigen-specific T-cells that recognized multiple GAD65- and preproinsulin-derived peptides and coexpressed CD25(+)CD134(+) were confined to patients (n = 32) and pre-diabetic probands (n = 5). Autoantigen-reactive T-cells in control subjects (n = 21) were CD25(+)CD134(-) and recognized fewer autoantigen-derived peptides. Insulin therapy did not induce CD25(+)CD134(+) T-cells in type 2 diabetic patients. The coexpression of CD25 and the costimulatory molecule CD134 on memory T-cells provides a novel marker for type 1 diabetes-associated T-cell immunity. The CD134 costimulatory molecule may also provide a novel therapeutic target in type 1 diabetes.