BACKGROUND: The evaluation of behavioral and psychological symptoms (BPSD) is important for the diagnosis and management of Alzheimer's disease (AD). Previous studies have reported the prevalence and severity of BPSD changes. However, these studies have mainly focused on the AD patients with mild to moderate severity. Our study investigated the BPSD in AD patients with more advanced stages and looked at the prevalence and severity of BPSD with the progression of disease. METHODS: One hundred thirty six patients with probable AD received the Korean version of the neuropsychiatric inventory along with the expanded version of the Korean Clinical Dementia Rating Scale (CDR) and the Korean version of the Mini-Mental State Examination (K-MMSE). RESULTS: The mean K-MMSE, CDR and NPI scores were 10.1 (SD=7.1), 2.0 (SD=1.5) and 32.7 (SD=26.2), respectively. CDR and K-MMSE scores did not correlate with the total NPI score but did correlate with some of the subscale NPI scores. Apathy had the highest relationship to CDR (r=0.39, p<0.01). Aberrant motor was most correlated with the total score of NPI (r=0.65, p<0.01). The mean number of positive NPI items was 4.3, which ranged from 2.1 (CDR 5 group) to 5.4 (CDR 2 group). The most frequent symptom was apathy and the least was euphoria. The severity of BPSD increased as the dementia severity increased to CDR 2 except apathy. Night-time behavior and anxiety were frequent in the early stages whereas apathy and aberrant motor were frequent symptoms in later stages. CONCLUSIONS: These observations suggest that BPSD is relatively independent of cognitive functions. The prevalence and severity of BPSD with the exclusion of apathy, increased as the dementia severity increased from CDR 0.5 to CDR 2 and then declined declined except apathy.