Although delusions are a frequent finding in patients with Alzheimer's disease (AD), their mechanism is not well known. We carried out Tc 99m HMPAO single-photon emission computed tomography studies in 16 AD patients with delusions and 29 AD patients without delusions comparable in age, years of education, duration of illness, and severity of dementia. Although we found no significant between-group differences in performance on neuropsychological tasks, AD patients with delusions had significantly lower mean cerebral blood flow in both the left and right temporal lobes.