The underlying pathology of Isolated Coronary Artery Ectasia (ICAE) still remains unknown; however, endothelial dysfunction (ED) may be responsible for the disease process. The aim of this study was to assess the endothelial functions of patients with ICAE in comparison to patients with normal coronary arteries (NCA) by using flow-mediated dilatation (FMD) in the brachial artery. Thirty patients with ICAE, and 30 control subjects with angiographically NCA were included in this study. The FMD was measured in all patients using a high-resolution ultrasonographic system. The results showed that the baseline brachial artery diameters in patients with ICAE were not statistically different from those in patients with NCA (4.3 ± 0.3 vs.4.2 ± 0.3 mm, respectively, p = 0.265). However, the forearm FMD of the patients with ICAE was significantly lower than the values in the control group (8.5 ± 4.0 vs. 13.7 ± 3.0%, p <0.001). In addition, patients with ICAE have significantly higher levels of low-density lipoprotein (LDL) cholesterol compared to the control group (114.5 ± 28.8 vs. 97.9 ± 31.5 mg/dL, p = 0.03). Therefore, we conclude that patients with ICAE have more severe ED than patients with NCA, which supports the hypothesis that atherosclerosis may be involved in the pathogenesis of coronary ectasia. Hyperlipidemia may have a key role in the disease process.