Prospective psychiatric evaluations were performed as part of a longitudinal study of 32 demented patients who met criteria for the histopathological diagnosis of Alzheimer's disease at the time of death. Psychosis and major depressive disorder emerged in 15 (47%) and 7 (22%) patients, respectively, none of whom had a history of either behavioral disorder prior to the onset of dementia. The prevalence of psychosis increased with increasing dementia severity, was associated with more rapid cognitive decline, and once present was often persistent. In this regard, the emergence of psychosis in the context of Alzheimer's disease appears to be a poor prognostic sign. In contrast to its relationship to cognitive impairment, psychosis was not associated with an increased mortality rate.