Aβ, a nearly insoluble peptide, is generally assumed to irreversibly deposit and accumulate as senile plaques (SP) during the course of Alzheimer's disease (AD). We have studied temporal neocortex of normal elderly subjects, AD patients, and elderly Down syndrome (DS) patients to determine whether Aβ accumulates with age or with increasing duration of illness. We measured the number, size distribution, and total area (amyloid burden) of Aβ immunoreactive deposits using computerized image analysis techniques. We found far fewer SP in normal control subjects than in AD patients, who in turn have fewer SP than elderly DS patients. No measure of Aβ correlated with age in the control subjects, nor duration of illness in AD or DS patients. These data indicate that Aβ may not continue to accumulate during these disease processes and support the view that the amount of Aβ observed at autopsy may reflect competing processes of deposition and resolution of amyloid.