Purpose: This study was conducted to assess the level of agreement among survey instruments used to study dementia and to determine the prevalence rate of suspected dementia using different instruments. Subjects and Methods: A total 171 subjects older than 65 in Gyeongju-si, Gyeongsangbuk-do, were surveyed from February to October 2003. The age, sex and educational level were examined through interview surveys, and cognitive function was evaluated using three survey instruments including MMSE-K, S-SDQ, and KDSQ. Results: The cognitive function scores obtained with the MMSE-K showed significant difference according to age. For sex and educational level significant differences were observed with all three survey instruments. The kappa index, the level of agreement between the questionnaires, was 0.302 between MMSE-K and S-SDQ, 0.401 between MMSE-K and KDSQ, and 0.762 between KDSQ and S-SDQ. The prevalence rate of suspected dementia based on the criteria suggested by MMSE-K, S-SDQ, and KDSQ was 27.5%, 15.2%, and 17.0% respectively. Conclusion: The results of this study suggest that caution is needed when comparing the measured cognitive function scores and analyzing the prevalence of dementia; this is because the prevalence of dementia and cognitive functions vary according to the questionnaires used.