The neuropathology of Down's syndrome at middle age is compared with that of Alzheimer's disease at that age, through a review of the published literature and from the author's personal observations. The pathological changes of Down's syndrome at middle age, i.e. the form and distribution of senile plaques and neurofibrillary tangles, and the pattern of involvement (atrophy) of neuronal systems are qualitatively the same as those of Alzheimer's disease at that age. Quantitative differences do occur and these may relate to biological or sociological variations inherent to the two parent populations. It is concluded that, in pathological terms, patients with Down's syndrome at middle age do indeed have Alzheimer's disease. Some ways in which a study of patients with Down's syndrome can give insight into the nature and development of the pathological changes of Alzheimer's disease are put forward and discussed.