The Neuropsychiatric Inventory (NPI) is a standardized, validated, and reliable tool to assess neuropsychiatric derangements in dementia patients. The aim of this study is to develop the Korean version of the NPI (K-NPI) and to test its reliability and usefulness in dementia patients. The subjects were 49 normal controls and 92 patients with Alzheimer's disease (43), vascular dementia (32), frontotemporal lobar degeneration (11), and other causes (6). Their caregivers familiar with the subjects' everyday behavior were interviewed with the K-NPI. In a subgroup (29/141) of the caregivers, the K-NPI was repeated for test-retest reliability, average of 23.1 days after the initial test. Prevalence rates of 12 behavioral domains in dementia patients were comparable to those of the original NPI; apathy was the most common and hallucination was the least common behavior. Total K-NPI scores correlated positively with dementia severity assessed with the Korean Mini-Mental State Examination. Test-retest reliabilities of frequencies and severities of all subscales were significantly high. Depression, anxiety, apathy, irritability, night-time behavior, and eating change were identified at very low rates in normal controls and were significantly less than those in dementia patients (p<0.001). The K-NPI, whose reliability and competency are comparable to those of the original version, may be a reliable and useful tool for measuring neuropsychiatric disturbances in Korean dementia patients.