The aim of this 12-month prospective study was to establish whether severe internal carotid artery stenosis is associated with faster progression of the cognitive impairment in patients with Alzheimer's disease (AD). Four hundred and eleven patients with AD underwent extracranial carotid Doppler ultrasound evaluation. Cerebrovascular reactivity to hypercapnia was measured by means of the breath-holding index (BHI) in those with severe carotid artery stenosis using transcranial Doppler ultrasonography. Cognitive status was quantified with the Mini Mental State Evaluation (MMSE). Ninety-eight patients had severe carotid artery stenosis, 41 right (group 1), and 57 left (group 2), while 313 had no significant stenosis (group 3). Group 1 and 2 patients showed an increased probability compared with group 3 patients to develop severe dementia (MMSE scores < 21) during the 12-month follow-up period: OR 2.36 (95% CI: 1.14-4.87) and OR 4.90 (95% CI: 2.65-9.04), respectively (p < 0.05, multiple logistic regression analysis). A BHI value ipsilateral to the stenosis < 0.69 predicted a worse MMSE score at 12 months irrespective of the side of the stenosis. These findings suggest that severe internal carotid artery stenosis can be considered as a marker of a faster rate of progression of the cognitive decline in AD. They also indicate that cerebral hemodynamic evaluation could be applied to identify patients at higher risk of rapid cognitive decline, who may benefit from aggressive treatment, and warrant investigation of the advantages of carotid revascularization procedures in these patients.