Objective: The aim of the present prospective clinical study was to investigate the possible effect of endothelial dysfunction in urolithiasis. Methods: The study included 92 patients older than 18?years. The patients were divided into 4 groups with 23 patients each as group 1: metabolic sydrome (MetS) (?) stone disease (SD) (?), group 2: MetS (?) SD (+), group 3: MetS (+) SD (?) and group 4: MetS (+) SD (+). C-reactive protein, homocysteine, uric acid, and creatinine levels were evaluated between the groups. Endothelial (dys)function was evaluated based on the brachial artery flow-mediated dilation (FMD) measurement. Results: The mean age was 41.9???10.2 (range, 18?62) years. Mean FMD value was 15.9???18.2% (range, 24.0?68.5%). A strong significant difference was found between group 1 and 2 (p?.001); group 1 and 3 (p?.001) and group 1 and 4 (p?.001) with regard to FMD. Conclusion: These results suggest that endothelial dysfunction plays an important role in the pathogenesis of urolithiasis in patients with MetS.