Aβ peptide deposits are observed in brain cortical and leptomeningeal microvessels in a few families, in patients with Alzheimer's disease and in cognitively normal elderly subjects. These deposits, which cause Aβ amyloid angiopathy, are usually associated with other lesions induced by Aβ peptide and tau pathologies. To investigate the consequences of cerebral amyloid angiopathy on arterial morphology and search for correlations with the degree of cognitive impairment, we carried out a prospective clinicopathological and morphometric study in 29 institutionalized elderly patients cognitively normal or affected with sporadic dementia associated with Alzheimer-type lesions, cerebral infarcts or both. We measured the external and internal diameters of arteries 40–120 μm wide, containing moderate or severe Aβ deposits, and of unaffected arteries in the temporal and frontal lobes. We found no differences in the mean external diameters. In contrast, the mean internal diameters of vessels with moderate Aβ deposits were smaller than those of unaffected vessels. Conversely, the internal diameters of severely affected vessels were larger than those of unaffected vessels. This suggests that arterial walls become thicker during the early stages of amyloid angiopathy, and the diameter of the lumen decreases, whereas during advanced stages, the walls become thinner and the lumen becomes larger. In addition, we assessed the overall severity of amyloid angiopathy. This showed that thinner arterial walls and the severity of amyloid angiopathy were correlated to dementia. In a multivariate model that integrates the other macroscopic and microscopic lesions that may be implied in the mechanism of cognitive impairment, the severity of amyloid angiopathy per se explained 10% of the variability in the cognitive impairment.