BACKGROUND: Obstructive sleep apnea (OSA) is known to be an independent risk for cardiovascular diseases. The carotid intima.media thickness (IMT) is a surrogate marker of subclinical atherosclerosis. The aim of this study was to measure the carotid IMT in subjects with or without OSA and quantify its correlation with OSA severity. METHODS: Subjects were categorized into OSA and non-OSA groups based on polysomnography findings. The carotid IMT was considered a marker of atherosclerosis. The independent relationship between carotid IMT and OSA severity was analyzed using multiple linear regression. The presumed confounders were age, sex, body mass index, blood pressure, blood lipid and glucose profiles, smoking and alcohol drinking statuses, medications, being overweight, and having hypertension, diabetes mellitus, or hyperlipidemia. RESULTS: The carotid IMT was larger in OSA subjects (n=82, 0.620+/-0.096 mm [mean+/-SD]) than in non-OSA subjects (n=22, 0.551+/-0.078 mm; p=0.006), and was independently correlated with OSA severity (p<0.001). However, it was not affected by any of the other variables investigated. CONCLUSIONS: Subjects with OSA showed an increased IMT, with the thickness being correlated with the OSA severity. This finding suggests that accelerated atherosclerosis in OSA is the mechanism linking OSA and future cardiovascular events.