The increased severity of obstructive sleep apnoea syndrome (OSAS) is associated with a parallel increase in the incidence of cardiovascular events. Whether the increased severity of OSAS is in fact associated with impaired arterial properties has never been thoroughly studied. In patients with OSAS who carry a high burden of cardiovascular risk factors, we investigated whether the severity of OSAS is associated with deterioration in the arterial properties, independent of classical cardiovascular risk factors. In 74 consecutive patients with OSAS, we non-invasively assessed, by means of tonometry and high-resolution ultrasound: carotid intima-media thickness (IMT), carotid diameter and plaques, carotid-femoral pulse wave velocity (PWV), central augmentation index (AI) and central blood pressures. The respiratory disturbance index was an independent predictor of IMT and PWV but not of carotid plaques, carotid diameter, AI or central blood pressures. Several parameters of nocturnal hypoxaemia were independently correlated with carotid IMT and PWV. In conclusion, arterial stiffening and thickening are modulated by the severity of OSAS, independently from age and cardiovascular risk factors.