Patients with presenile and senile forms of Alzheimer's disease showed increased silicon (Si) in the isolated cores and in the rims of the senile plaques. The increased Si was substantially above background (3 to 50 times). Silicon was not increased as much in patients having Alzheimer changes as part of Down's syndrome. Cortical neurons in Alzheimer's disease showed no increase in Si compared with control neurons. However, Alzheimer neurons with neurofibrillary tangles had more Si than did those neurons without tangles. Silicon was also increased in corpora amylacea of two of three patients with Alzheimer's disease. The increase in Si in neurons with the neurofibrillary change was proportionately greater than that of iron. Indeed, there was no consistent increase in iron in Alzheimer's neurons. The present findings point to a discrete and focal increase in Si. The precise relationship between Si, ageing, and Alzheimer's disease remains to be determined.