Objectives: To evaluate endothelial function in a large cohort of children clinically referred for suspected obstructive sleep apnea syndrome (OSAS), and to identify risk factors contributing to the presence of endothelial dysfunction (ED). Methods: Habitually snoring children (age range, 3-11 years) were recruited. All participants underwent overnight polysomnography (PSG). Endothelial function test used peripheral arterial tonometry (PAT) to derive the reactive hyperemia index (RHI). Subjects were then divided into mild OSA, moderate-severe OSA and primary snorers (PS), according to their obstructive apnea-hypopnea index (OAHI). Results: A total of 355 subjects were recruited. There were no differences in age, gender, or BMI z score among the three groups. Both mild and moderate-severe OSA groups had lower RHI than PS (P < 0.001, P = 0.001, respectively). Linear regression analysis revealed that RHI was positively correlated with age (r = 0.17, P = 0.002), BMI z score (r = 0.14, P = 0.008) and oxygen saturation nadir (r = 0.15, P = 0.006), but negatively correlated with oxygen desaturation index (ODI3%; r = -0.19, P = 0.001) and respiratory-related arousal index (ArI-resp) (r = -0.24, P < 0.001). In stepwise regression analysis, age, BMI z score, and ArI-resp were independently associated with endothelial function (r = 0.34, P < 0.001). Conclusion: Children with OSA are at increased risk for abnormal endothelial function than habitually snoring children. Furthermore, in addition to age and BMI, which are well-established factors affecting endothelial function, both intermittent hypoxia and sleep fragmentation during sleep also emerge as candidate risk factors contributing to endothelial dysfunction in snoring children. Clinical trial: Follow up of PS and OSAHS in Chinese children, https://clinicaltrials.gov/, Clinical number: NCT02447614. (c) 2018 Elsevier B.V. All rights reserved.