The goals of this article are to (a) establish the concurrent and clinical validity of the Global Deterioration scale in assessing cognitive functions and stages of dementia among centenarians, (b) identify the prevalence of all-cause dementia in representative samples of centenarians, and (c) demonstrate how variations in sample demographic characteristics could significantly affect estimates of dementia prevalence. A quarter of the 244 centenarians in a population-based sample had no objective evidence of memory deficits. Another quarter showed signs of transient confusion, and about half showed classical behavioral signs of dementia with about 15% in each of Global Deterioration scale stages 4-6 and about 5% in the most severe stage 7. Variations in age, gender, race, residence status, and education of the study sample as well as criteria used for dementia rating were found to affect prevalence.