The prevalence and severity of calcification in the basal ganglia (BGC) has been examined histopathologically in 194 patients divided into ten diagnostic categories. The prevalence and severity of BGC was greater (for age) in Down's syndrome and in patients under 75 years of age with Alzheimer's disease. The severity, but not the prevalence, of BGC was greater in Down's syndrome than in patients of similar age with Alzheimer's disease. Both the prevalence and the severity of BGC in patients over 75 years of age with Alzheimer's disease were as expected for age alone. The increased prevalence and severity of BGC in Down's syndrome and in younger patients with Alzheimer's disease appeared not to be related to the presence of dementia or degenerative disease per se, nor was it affected by the presence of cerebral infarction. BGC may result from an age-related disturbance of the structure of arteries within the globus pallidus, which is accelerated (or occurs prematurely) in Down's syndrome and in younger patients with Alzheimer's disease, but probably does not form part of that spectrum of changes that constitutes the pathological basis of Alzheimer's disease.