The brains of patients with a clinical history of Alzheimer's disease were collected for study. In 57 cases the diagnosis of Alzheimer's disease was confirmed histologically. The disease was of senile onset (over the age of 65 years) in 40 cases, of presenile onset in 8 cases, and there were 9 cases of Down's syndrome with Alzheimer's disease. In each case the cerebellum was examined for the presence of amyloid plaques. Cerebellar amyloid plaques were found in 31 of the 57 cases (52%) and were present in all 3 clinical groups. Amyloid involvement of the cerebral and cerebellar arteries was marked in many but not all of the cases with cerebellar amyloid plaques. The significance of cerebellar plaques is uncertain, but their presence in Alzheimer's disease is by no means as rare as previously thought. The findings are described and some preliminary clinicopathological correlations are made.