Epidemiological studies have suggested a pathophysiological link between obstructive sleep apnea syndrome (OSAS) and Alzheimer's disease (AD). The mechanism by which sleep disturbance can affect cognitive impairment is not clear. The aim of this study was to investigate whether AD patients with OSAS have an impairment in cerebrovascular disease markers. We included 69 patients without OSAS and 93 patients with OSAS. They underwent an ultrasonographic assessment of common carotid arteries intima-media thickness (IMT) and carotid plaque index. Cerebrovascular reactivity to hypercapnia in the middle cerebral arteries was calculated with the Breath-Holding Index (BHI). Pathological values of IMT and BHI were significantly associated with the presence of OSAS (IMT > 1.0 mm: OR 2.98, 95%CI: 1.37–6.46; p < 0.05; BHI < 0.69: OR 5.25, 95%CI: 2.35–11.74; p < 0.05, multivariate adjusted analysis). Furthermore, the extent of cerebrovascular impairment was correlated with the severity of OSAS. The finding of alterations of cerebral vessel functional and anatomic status in AD patients with OSAS suggests the potential for effective treatment for sleep-related disturbances in a subgroup of AD patients.