Although impaired endothelial function is well known in patients with diabetes mellitus (DM), the precise mechanism and the factors that contribute to this dysfunction remain to be clarified. The authors examined the effect of acute hyperglycemia on endothelium-dependent vasodilation in patients with DM or impaired glucose metabolism in vivo by plethysmography. In eight patients with diabetes mellitus or impaired glucose metabolism, the vasodilatory response to acetylcholine at infusion rates of 7.5, 15, and 30 microg/min was studied in the fasting state and at two levels of hyperglycemia, which were achieved by the infusion of glucose, insulin, and somatostatin. The vasodilatory response to acetylcholine was measured by calculating the forearm blood flow ratio (FBFR), defined as the measured forearm blood flow at a specific acetylcholine infusion rate divided by the baseline forearm blood flow without acetylcholine infusion. The induction of hyperglycemia resulted in a significant reduction in FBFR for all rates of acetylcholine infusion. The FBFR at an acetylcholine infusion rate of 7.5 microg/min was 1.13 +/- 0.07 and 1.19 +/- 0.06 for stages 1 and 2, respectively, compared with 1.36 +/- 0.08 for stage 3 (p < .05). In addition, at an acetylcholine infusion rate of 30 microg/min, the reduction in FBFR was associated with the degree of hyperglycemia (3.72 +/- 0.51, 2.98 +/- 0.42, 2.42 +/- 0.32 for stages 1, 2, and 3 glucose levels, respectively, at an acetylcholine infusion rate of 30 microg/min, p < .05 by analysis of variance [ANOVA]). The results show that the induction of hyperglycemia results in a significant attenuation of endothelial function, and suggests the importance of hyperglycemia in the development of endothelial dysfunction observed in patients with DM or impaired glucose metabolism.