The aim of this study was to investigate behavioral and psychological symptoms of dementia (BPSD) measured by caregiver-administered neuropsychiatric inventory (CGA-NPI) as a function of dementia severity in early onset (EOAD) versus late onset Alzheimer's disease (LOAD). A total of 113 patients with AD consisting of 49 patients with EOAD and 64 patients with LOAD were enrolled consecutively. General cognitive function and severity of dementia were assessed by the Korean version of mini-mental status examination and clinical dementia rating (CDR), respectively. In the mild stage (CDR 0.5-1), LOAD patients had a significantly higher total CGA-NPI score than EOAD patients. Subgroup analysis demonstrated that disinhibition and night-time behavior were more common and severe in the LOAD group than the EOAD group. However, in the moderate to severe stage (CDR 2-3), EOAD patients had a significantly higher total CGA-NPI score with higher subscores in hallucination, agitation/aggression, irritability/lability, aberrant motor behavior, and appetite/eating change. This study suggested that the heterogeneity of BPSD in AD might be accounted for by dementia severity as well as age at symptoms onset.