Aim: We investigated differences in the prevalence and severity of 10 neuropsychiatric and behavioral symptoms according to apolipoprotein E (APOE) genotype and dementia severity in Alzheimer disease (AD). Methods: Neuropsychiatric and behavioral symptoms of 110 AD patients were assessed using the Neuropsychatric Inventory. Dementia severity was assessed using the Mini Mental State Examination (MMSE). Results: There were 27 APOE-ε4-negative patients, 65 heterozygous patients and 18 homozygous patients. There was a significant association between the number of APOE ε4 alleles and prevalence and severity of neuropsychiatric and behavioral symptoms that was mainly attributable to delusions and agitation/aggression, which were more common and severer among homozygous APOE ε4 carriers. In addition, the presence of hallucinations, anxiety, apathy and aberrant motor behavior increased with deteriorating MMSE score, independently of APOE ε4 status. Conclusions: The present study showed that the APOE ε4 genotype modifies neuropsychiatric and behavioral phenotype in AD. In particular, it was shown that delusions and agitation/aggression were more common and severer among homozygous APOE ε4 carriers than among heterozygous or APOE-ε4- negative patients.