Cerebral amyloid angiopathy (CAA) was found in 57% of 123 autopsy brains removed from patients aged 59-101 years. The incidence of CAA increased with age. CAA was seen most frequently in the occipital cortex. Immunohistochemically, amyloid of CAA was positive for amyloid P component and negative for human AA protein and human prealbumin. The presence and severity of CAA were significantly correlated with the number of senile plaques and neurofibrillary tangles. The incidence of CAA in 17 patients with dementia of Alzheimer type (DAT) was estimated to be 88% and was significantly higher than that in 26 patients with dementia of non-Alzheimer type. CAA had a pathogenetic relationship with both brain ageing and DAT. Lobar cerebral haemorrhage was found in 3 patients with CAA of marked or moderate degree. Lobar cerebral haemorrhage in the aged and in patients with DAT suggest the presence of CAA.