BACKGROUND: Dementia Screening Questionnaire is sensitive to detect early dementia patients and is not influenced by age and educational level. We conducted a case-control study to assess the validity and reliability of the Korean Dementia Screening Questionnaire (KDSQ) for the diagnosis of early dementia. METHODS: We developed KDSQ in a semi-structured manner with the questionnaire of 15 items about cognitive dysfunctions and 5 items of ischemic and depression scales respectively to evaluate the possibility of vascular dementia and degree of depression. It was adminis-trated to informants of 72 dementia patients (39 Alzheimer's disease, 33 vascular dementia) with Clinical Dementia Rating (CDR) scales of 0.5, or 1.0 and to those of 66 hospital control subjects. Test-retest data for the KDSQ were obtained from informants within a time interval of 20.82 days. RESULTS: The KDSQ was not influenced by education level, age, or sex. KDSQ score correlated with the K-MMSE (r=-0.75) and Short form Samsung Dementia Questionnaire (r=0.80). The area under the receiver operating characteristic curve for the KDSQ was 88.9% (S.E.=0.028). KDSQ depression score was correlated with Geriatric Depression Scale (r=0.34). With respect to a diag-nosis of dementia, the KDSQ (cut-off point 6) had a sensitivity of 79% and a specificity of 80%. The KDSQ was found to have a high test-retest reliability (r=0.81) CONCLUSIONS: The KDSQ has high validity and reliability for the diagnosis of early dementias. It may be a useful tool to screen early stage of dementias.