Objective: To evaluate the functional changes of the arterial endothelium and smooth muscle after high-voltage electrical injury (HVEI) using flow-mediated dilation (FMD) and nitrate mediated dilation (NMD). Methods: Twenty-five male patients with arms injured by a current of more than 20,000 volts were enrolled the study. FMD and NMD were measured on the brachial artery within 48 hours after HVEI. Follow-up FMD and NMD were evaluated 6 weeks later. The variables were compared with a control group of healthy men matched for age and body mass index. Results: A significant decrease in the initial FMD was seen in the HVEI group compared with the control group (2.1짹1.2% vs. 13.6짹3.4%, p<0.01). At the 6 week follow-up, the FMD of the HVEI group was significantly improved compared with the initial FMD (2.1짹1.2% vs. 5.1짹2.1%, p<0.01), but it was still decreased compared with the FMD of the control group (5.1짹2.1% vs. 13.6짹3.4%, p<0.01). A significant decrease in the NMD was seen both initially and at the 6 week follow-up compared with the NMD of the control group (7.3짹4.7% vs. 20.4짹4.1%, p<0.01 and 11.4짹6.7% vs. 20.4짹4.1%, p<0.01, respectively). Multivariate logistic regression analysis identified that only the initial FMD value was negatively associated with no improvement in the 6 week FMD (less than 1% improvement) at a marginal significance (RR 0.25; 95% CI 0.06-1.06, p=0.06). Conclusions: After HVEI, the endothelial and smooth muscle functions of the brachial artery were significantly decreased for 6 weeks. Long term cautious care is needed for all victims of HVEI, because of increased risk of thrombosis or stenosis in the injured arm.