A cross-sectional, population-based, 2-stage prevalence study was conducted in a sample of 1019 community-dwelling persons over the age of 70 years living in Istanbul. In the first phase, participants were screened with the Mini-Mental State Examination for evidence of cognitive impairment. In the second phase, 79% of those who screened positive (n = 322) and 9% of screen-negatives (n = 63) underwent a standardized diagnostic workup. Diagnosis of dementia and Alzheimer's disease (AD) was made according to established criteria. Ninety-three cases of dementia were identified, 58 of whom were diagnosed with probable AD. Based on these numbers, the prevalence rates of probable AD and dementia were calculated to be 11.0% (95% CI, 7.0% to 15.0%) and 20.0% (95% CI, 14.0% to 26.0%), respectively, in this population. Prevalence rates of dementia and AD in Istanbul, Turkey, are comparable with those seen in the Western world.