['Non-invasive detection of vascular dysfunction in the early stage is clinically important in patients with sleep apnea syndrome (SAS). Flow-mediated dilatation (FMD) is a novel clinical marker of endothelial function. However, it is not clear whether this is useful in the SAS patient.', 'Echocardiographic parameters and FMD were measured in 129 patients with SAS. Apnea-hypopnea index (AHI) was defined by polygraphy, and patients were divided into the two Groups: Group A (moderate-severe SAS: AHI≥ 15 times/hr, n=93) and Group B (mild SAS: AHI 5-15 times/hr, n=36).', 'There were no significant differences in echocardiographic parameters between the two groups. However, FMD was significantly lower in Group A than in Group B (3.5±1.6 vs. 7.8±3.1, P< 0.01).'] Does flow-mediated dilatation identify impaired endothelial function in patients with sleep apnea syndrome?