Immunohistochemical and immunoblotting studies with an antibody against cyclo-oxygenase 2 (COX2) were performed in the cerebral cortex of patients with Down's syndrome (DS) and dementia of Alzheimer type (DAT). A high level of COX2 expression was observed in DAT and older DS patients, specifically localized in neurones with neurofibrillary tangles (NFT) and damaged axons. Furthermore, immunohistochemical study of patients with DS of varying age showed that the induction of COX2 correlated well with the appearance of NFT as well as with ageing. These findings demonstrated the induction of COX2 in DAT and DS, which may lead to the production of free radicals and may be causally related to neuronal degeneration.