Cerebellar amyloid deposits in Alzheimer's disease were studied by immunocytochemistry and with a series of antibodies that recognize human microglia, including anti-HLA-DR, LN-1, Leu-M5 and leukocyte common antigen. Microglia formed a dense reticular array throughout the cerebellum in areas with and without amyloid deposits. In areas with compact and reticular amyloid deposits, microglia had morphological features consistent with activation, such as cytoplasmic swelling and shortening and thickening of cell processes. In areas with diffuse amyloid deposits, microglia had delicate and highly ramified processes. Nevertheless, microglial cells or their processes were detected in association with amyloid deposits of all morphological types. These results raise the possibility that microglia may play a fundamental role in the pathogenesis of amyloid deposition in the cerebellum in Alzheimer's disease.