Background: Moderate-severe obstructive sleep apnea (OSA) is associated with endothelial dysfunction, increased arterial stiffness and hypertension. It is not known if minimally symptomatic OSA is also associated with impaired vascular function. Methods: In 64 patients (7 females) with minimally symptomatic OSA (oxygen desaturation index 23.1 SD15.6, Epworth sleepiness score 8 SD3.8), and 15 matched controls without OSA, endothelial function was assessed by ultrasonographic measurement of flow-mediated dilatation (FMD), and by applanation tonometry derived pulse wave analysis (forearm ischemia and salbutamol induced changes in augmentation index, AI_x). Arterial stiffness was assessed by AI_x and ambulatory blood pressure (ABP) was measured during one week. Results: In patients with OSA, FMD was significantly lower than in controls (5.0 SD2.7% and 7.5 SD3.3% respectively, p=0.003). AI_x was significantly higher in the OSA group compared to the control group (26.0 IQR19.0-29.5% and 21.0 IQR8.0-27.0%, respectively, p=0.04). Change of AI_x after both forearm ischemia/salbutamol was significantly smaller in patients with OSA (-2.0 IQR-5.0 to +4.0 and -3.0 IQR-7.0 to 0.0% respectively), than in controls, (-6.0 IQR-8.0 to -5.0 and -7.0 IQR-10.0 to -3.0%, p=0.005 and p=0.04 respectively). ABP was similar (97.6 SD7.9 and 94.8 SD7.4 mmHg, OSA and control groups respectively, p=0.21). Conclusions: In patients with minimally symptomatic OSA, diverse properties of endothelial function are impaired and arterial stiffness is increased. Although this was not associated with a significantly increased ABP, the findings suggest that patients with minimally symptomatic OSA are at increased cardiovascular risk.