Obstructive sleep apnea (OSA) and metabolic syndrome (MetS) are associated with increased cardiovascular morbidity and mortality. Increased homocysteine is suggested as an independent risk factor for atherosclerosis and cardiovascular disease but remains disputed in OSA. We assessed polysomnography, carotid intima-media thickness (CIMT) and biology in 35 MetS patients, according to the presence (OSA+MetS; n=26) or the absence of OSA (MetS; n=9). In OSA+MetS patients, homocysteine levels were increased compared to MetS subjects (12.8 ± 3.8 vs. 9.5 ± 2.5 μmol/L; P=0.026). In the whole population, homocysteine correlated with apnea-hypopnea index (AHI) (r=0.522; P=0.001) and CIMT (r=0.376; P=0.026). Homocysteine was negatively correlated with plasma thiols (r=-0.406; P=0.017) and positively with urinary 15-F2t-isoprostanes (r=0.347; P=0.044). Multivariate regression analysis revealed that AHI (β=0.559; P