Cotton wool plaques (CWP) are large, ball-like plaques lacking dense amyloid cores that displace adjacent structures. They were first described in a Finnish kindred with early-onset Alzheimer disease (AD) with spastic paraparesis due to a presenilin-1 delta9 mutation. We describe a case of sporadic late-onset AD with numerous neocortical CWP as well as severe amyloid angiopathy and marked leukoencephalopathy, compared with 16 cases of late-onset AD with similar degrees of amyloid angiopathy and leukoencephalopathy. The cases were studied with histologic methods and with single and double immunostaining for beta-amyloid (Abeta), paired helical filaments-tau (PHF-tau), neurofilament (NF), glial fibrillary acidic protein (GFAP), HLA-DR, and amyloid precursor protein (APP). We found that CWP were well-circumscribed amyloid deposits infiltrated by ramified microglia and surrounded by dystrophic neurites that were immunopositive for APP, but only weakly for NF and PHF-tau. Abeta1-12 was diffuse throughout the CWP, while Abeta37-42 was peripherally located and Abeta20-40 more centrally located. Two of the 16 late-onset AD cases also had CWP, but they were also admixed with diffuse plaques and plaques with dense amyloid cores. Pyramidal tract degeneration was not a consistent finding or a prominent feature in any case. The results suggest that CWP are not specific for early-onset familial AD with spastic paraparesis.