Purpose: The purpose of this study was to investigate the ultrastructural corneal changes of chronic diabetic monkeys and explore the relationship between advanced glycation end products and ultrastructural changes in diabetic corneas. Methods: A total of 8 cynomolgus monkeys were used in this experiment. Four monkeys were induced into insulin-dependent diabetes mellitus for 4 years. Four age-matched healthy monkeys were used as the controls. Ultrathin sections obtained from the corneas were examined by transmission electron microscopy. Results: Advanced glycation end product immunoreactivity was observed in the epithelial cells, epithelial basement membrane, and stromal keratocytes of diabetic corneas, whereas advanced glycation end product immunoreactivity was not found in the corresponding area in normal corneas. Abnormal collagen fibril bundles of variable thickness were identified in corneal stroma in all diabetic monkeys. Epithelial and endothelial cell degeneration was also observed in 1 diabetic monkey. Conclusions: Abnormal aggregates of collagen fibrils in stromal matrix were common among long-term diabetic monkeys, and the formation of the abnormal collagen fibril aggregates might result from excessive nonenzymatic glycosylation.