Background CPAP treatment for symptomatic OSA improves surrogate markers of cardiovascular risk, such as endothelial function and arterial stiffness, and may reduce actual cardiovascular events. Minimally symptomatic OSA is far more prevalent than symptomatic OSA but the effects of CPAP on endothelial function and arterial stiffness in minimally symptomatic patients are not known. Methods In two centres taking part in the MOSAIC trial (Oxford and Taunton), 253 patients with minimally symptomatic OSA (ODI>7.5 h) were randomised to either 6 months of CPAP or supportive care. 245 patients had measurements of arterial stiffness by pulse wave analysis at baseline (augmentation index, AIx) and in 64 patients endothelial function was assessed by brachial artery flow-mediated dilatation (FMD, expressed as % change from baseline arterial diameter) measurements by ultrasonography. Multivariable analyses adjusting for baseline FMD or AIx, ODI and Pocock vascular risk score (age, sex, systolic BP, smoking, diabetes, cholesterol, height, creatinine, LVH, previous MI or stroke) were performed to assess the effect of CPAP treatment on FMD and AIx. Results Of the 245 patients 8 withdrew or were lost to follow-up and in 8 patients pulse wave analysis was not possible at 6 months. All 64 patients who had FMD measurements at baseline attended follow-up measurements. Baseline characteristics of 229 patients with complete data on pulse wave analysis are shown in the table (values are mean (SD) were applicable). CPAP improved endothelial function (FMD at follow-up +1.97% with CPAP compared to control group, 95% CI +0.84 to +3.09%, p=0.001), but there was no evidence of an effect on arterial stiffness (AIx at follow-up −0.56% with CPAP compared to control group, 95% CI −2.87 to +1.75, p=0.64). CPAP improved daytime sleepiness as assessed by the Epworth sleepiness score (mean (SE) change −1.91 (0.30) with CPAP; control group +0.08 (0.26), mean difference −1.99, 95% CI −2.77 to −1.21, p t -test). Conclusions 6 months of CPAP is associated with improved endothelial function, but does not reduce arterial stiffness in minimally symptomatic OSA. Thus patients with minimally symptomatic OSA may benefit from CPAP therapy in terms of cardiovascular risk reduction.