A prospective cross-sectional investigation examining the relationship of neuropsychological and behavioral changes to the occurrence of delusions in dementia syndromes was conducted. Nineteen patients had Alzheimer's disease (AD), and 14 had multi-infarct dementia (MID). Patients with and without delusions were compared with regard to demographic characteristics, neuropsychological and neurological features, and a variety of behavioral disturbances. Delusional patients were more aggressive and exhibited more severe activity disturbances than nondelusional patients. Delusional patients were more severely cognitively impaired, but the neuropsychological differences between the two groups were not outstanding. These observations suggest that specific neuropsychological deficits are not compelling predictors of delusions and that delusional patients are more behaviorally disturbed than those without delusions. It is hypothesized that delusions are independent noncognitive manifestations of the neurobiology of AD and MID.