The aim of the present study was to investigate the impact of acute hyperglycaemia on endothelial function in both normal-weight and overweight children. A total of 16 overweight [BMI (body mass index) ≥85th percentile] and 15 normal-weight (BMI <85th percentile) children were evaluated for FMD (flow-mediated dilation) at baseline and 30, 60 and 120 min after glucose ingestion. At 15 min following the measurement of the final FMD, 0.3 mg of sublingual nitroglycerine was administered and the brachial artery was imaged in order to assess endothelium-independent dilation. By design, the overweight children were significantly heavier (63.2±4.6 compared with 41.3±2.5 kg; P =0.0003) and had a greater percentage body fat (43.9±1.8 compared with 23.8±2.05%; P <0.0001) than the normal-weight children. The area under the curve in response to glucose administration was significantly ( P <0.0001) greater in the overweight group for both glucose and insulin. The FMD area under the curve was not significantly different at baseline or between time points after glucose ingestion, nor was there a difference in response between the two groups. Endothelium-independent dilation in the normal-weight group was significantly greater compared with the overweight group (26.7±1.6 compared with 20.2±2.0% respectively; P =0.019). In conclusion, these results suggest that acute elevation of glucose and insulin in overweight and normal-weight children are not associated with impairment in endothelial function.