Aim: Although underlying mechanisms of coronary artery ectasia (CAE) are clearly unknown, endothelial dysfunction may be responsible for the coronary ectasia. Asymmetric dimethylarginine (ADMA) is the principal endogenous inhibitor of nitric oxide synthase and elevated levels of ADMA is considered to be a marker of endothelial dysfunction and increased risk of cardiovascular disease. Accordingly, we investigated serum ADMA levels in patients with CAE in comparison with participants having normal coronary artery. Method: This study consisted of 41 consecutive patients with angiographically proven normal coronary arteries with CAE and 48 controls with normal coronary arteries without coronary artery ectasia undergoing cardiac catheterization. Serum levels of ADMA were measured by Enzyme Linked Immunosorbent Assay. Result: Serum level of ADMA was significantly higher in CAE patients compared with controls (1.9±0.9 μmol/l vs. 1.1±0.7 μmol/l, respectively, p= 0.01). Serum ADMA levels was an independent predictor for CAE (OR= 1.486 [95% confidence interval (CI): 0.978–2.054; p= 0.03]). ADMA levels correlated with diffuse ectasia (p= 0.04). Conclusion: Our findings provide evidence to support the hypothesis that endothelial dysfunction may be an important factor in the pathogenesis of CAE.