In a prospective, open-label, 12-week pilot study in outpatients with probable Alzheimer disease (AD) with psychosis or aggressive behaviors, we used the Alzheimer's Disease Assessment Scale-cognitive subscale (ADAS-cog) and the Neuropsychiatric Inventory (NPI) to evaluate the cognitive and behavioral effects of quetiapine. After receiving doses ranging from 50 to 150 mg, patients given quetiapine showed a significant decrease of delusions, aggression, and overall behaviors based on NPI scores at 6 and 12 weeks. ADAS-cog scores did not show a significant change over 12 weeks. This study provides initial evidence that quetiapine does not significantly worsen cognition in AD outpatients.