The aim of this study was to evaluate whether abnormal endothelial function is present in early stages of diabetes, such as impaired glucose tolerance (IGT) and impaired fasting glucose (IFG). Endothelial function was assessed by measuring flow-mediated dilatation and nitrate-induced dilatation of the brachial artery using high-resolution ultrasound. Fasting serum lipid levels were determined, and glucose and insulin values in response to a 75-g oral glucose load were also measured. The results showed the following new findings: (1) compared with subjects with normal glucose tolerance, those with IFG and IGT had impaired flow-mediated dilatation, more remarkable in subjects with type 2 diabetes mellitus than those with IFG and IGT, and (2) flow-mediated dilatation was inversely and strongly related to the extent of hyperglycemia. In conclusion, endothelial dysfunction is present in subjects with IGT and IFG, indicating endothelial damage in these stages.