Background: Alpha-lipoic acid (ALA) is known to improve endothelial function in diabetes. But, effect of ALA to endothelial function is unknown in hypertensive patients. The aim of this study is to investigate whether ALA improves the endothelial function in diabetic and hypertensive patients. Methods: This study included 40 patients who were treated with ALA (Group I: 20 diabetic patients, 54.7짹10.0 years, Group II: 20 hypertensive patients, 57.7짹9.7 years). ALA was administered 600mg during the first four weeks and 1200 mg during the second four weeks. Clinical characteristics, endothelial function assessment at brachial artery for flow mediated vasodilation (FMD) and inflammatory marker (high sensitive C-reactive protein: hsCRP) were measured at baseline, at 4 weeks and 8 weeks after ALA administration. Results: Changes of clinical characteristics (body mass index, total cholesterol/HDL-cholesterol and hsCRP) were not different in both groups. FMD was significantly improved at 8 weeks after ALA therapy in both groups (Group I - baseline: 4 weeks:8 weeks = 4.1짹3.4:6.4짹2.2:8.0짹2.8, Group II - baseline:4 weeks:8 weeks = 5.5짹3.8:7.4짹3.4:9.3짹2.8, p<0.05). Conclusion: ALA improves the endothelial function in both diabetic and hypertensive patients.