Pancreatic tssue was obtained under peritoneoscopy from a 33-year-old man with recently occurring Type I (insulin-dependent) diabetes. He had ICA, CF-ICA and ICSA, and increased DRpositive activated T-lymphocytes. Islets were atrophic and insulin-containing cells were reduced in number, although lymphocytic infiltration was not observed. Most of the islets were positive for DR antigens; DR molecules were detected preferentially on insulin-containing cells. In addition to endocrine cells, pronounced expression of DR antigens was observed on the hypertrophic edothelium of capillaries in and around islets. The expression of DR antigens may be involved in the development of autoimmune reactions to islet cells. Augmented HLA-DR antigens on endothelial cells suggests a possible role of capillary alterations in islet cell damage in Type I diabetes.