BACKGROUND: Multiple studies in patients with diabetes demonstrate impaired endothelial-dependent vasodilation. But the mechanisms of vascular dysfunction in type 2 diabetes are still controversial. Some risk factors, such as dyslipidemia, hypertension and obesity, are commonly associated with type 2 diabetes. These risk factor may cause endothelial dysfunction. And hyperglycemia may have a specific role in the increased risk of vascular complications in diabetes but it remains unclear. The purpose of this study was to examine whether endothelial dysfunction occurs when acute hyperglycemia is induced by oral glucose loading. METHOD: Using the high-resolution ultrasound, we measured flow-mediated vasodilation (endothelial dependent vasodilation: FMD) during oral glucose tolerance test in 11 men (mean age: 59+/-5 years) with type 2 diabetes without chronic diabetic complications. For statistical analysis, we used paired t-test, generalized linear method (GLM) to compare FMD before and after glucose loading. RESULT: Flow-mediated vasodilation was diminished after glucose loading (13.2+/- 6.4%, 7.3+/-3.3*%, 12.8+/-5.6%, in fasting, at 1- and 2-h, respectively; *p<0.001 vs fasting). Superoxide anion formation by neutrophils was increased after glucose loading (4.65+/-2.8, 6.17+/-2.2, in fasting, at 1-h respectively: p<0.05 vs fasting)( 10-7nmol/106cells/30min). Endothelial independent vasodilation was not significantly affected by glucose loading. The concentration of triglyceride were not changed after glucose loading. CONCLUSION: This study shows that acute hyperglycemia induced by 75 gm oral glucose intake results in endothelial dysfunction. These results suggest that prolonged and repeated hyperglycemia may play an important role in the developement and progression of vascular complication in diabetes.