The diameters and densities of capillaries and arterioles in the hippocampal cortex of normal subjects and patients with Alzheimer's dementia were measured in thick celloidin sections stained for alkaline phosphatase. Microvascular diameters in general are affected more by age than by the presence of dementia of the Alzheimer type. The diameter of both capillaries and arterioles increases significantly with age. The density of capillaries decreases whereas that of the arterioles increases significantly. The capillary changes suggest that a reduced exchange potential accompanies ageing. In brains of people with Alzheimer's disease the overall capillary diameters and densities do not differ from those of age-matched controls. Regional changes may, however, be important: those hippocampal zones showing the greatest severity of or increment in nerve cell lesions do correspond to those having the highest levels of or increase in capillary density and the greatest decrease in diameter, suggesting a direct association between neuronal susceptibility to Alzheimer changes and degree of regional blood supply. Capillary surface areas, volumes and area/capillary volume ratios support the possibility of this relationship. Neurofibrillary tangles and granulovacuolar degeneration do not correlate equally with the degree of capillary "irrigation"; tangles are more closely related to these morphological vascular parameters.