Introduction: Assessment of endothelial function in type 2 diabetic patients may be important in terms of reducing cardiovascular mortality. In this study, the relationship between hyperuricemia and endothelial dysfunction was investigated in type 2 diabetic patients. Method: The study consisted of type 2 diabetic patients with serum uric acid level greater or equal to 5.5 mg/dl as the experimental group and type 2 diabetic patients with serum uric acid level less than 5.5 mg/dl as the control group. Malondialdehyde (MDA) level as a marker of oxidative stress and superoxide dismutase (SOD), glutathione peroxidase (GPO), and catalase levels as markers of antioxidant system were analyzed. Endothelial function was assessed by measurement of flow-mediated dilatation (FMD) in forearm. Results: Demographic characteristics were similar in both groups. No significant difference was seen between the two groups in terms of MDA, SDO, GPO, or catalase levels. The study group's FMD was found to be significantly decreased compared to the control group (7 +/- 4.5% versus 9.8 +/- 5.1%, P = 0.02). Uric acid levels, body mass index, systolic blood pressure, diastolic blood pressure and microalbuminuria were negatively correlated with FMD in the one-way analysis. However, in the multivariate analysis, only uric acid was found to be negatively correlated with FMD significantly (OR 0.37, 95% Cl 0.17 to 0.77, P = 0.008). Conclusion: Hyperuricemia in type 2 diabetic patients contributes to endothelial dysfunction by reducing FMD. This effect of uric acid cannot be explained by oxidative stress according to this study. Serum uric acid may be a new target for prevention of complications.