Diffuse plaques are the earliest Alzheimer-type lesions in Down's syndrome and are a putative marker for the preclinical stage of Alzheimer's disease (AD). As a cerebral cortical cholinergic deficit is one of the characteristics which defines AD, we examined the brains of individuals who had died without a history of neurological disease to determine whether this deficit is present in association with diffuse plaques. Of the 24 cases collected, 14 were older than 60 years of age (mean 69.2 years) and 10 were younger (mean 29.6 years). Of the 14 older cases, 9 had diffuse plaques in the entorhinal cortex (ECx) and/or inferior temporal gyrus (ITG). The older cases were divided into two groups (plaque-positive or plaque-negative cases). These groups did not differ significantly with respect to age, post-mortem delay, synaptophysin immunoreactivity or neurofibrillary tangle density. Cholinergic fibre densities were estimated in sections stained using acetylcholinesterase (AChE) enzyme histochemistry. Mean AChE fibre density was decreased in both the ITG and ECx (approximately 30% and 50% depletion, respectively) in the plaque-positive group compared to the plaque-negative group and in both areas the mean fibre density of the plaque-positive group was about 50% of that in the younger group. These results suggest that diffuse plaques in the non-demented elderly are associated with an accelerated age-related cortical cholinergic deficit and, therefore represent the preclinical stage of AD.