The prevalence of dementia increased in women (from 31% to 46%) but not in men (from 27% to 25%) in a representative birth cohort followed from age 85 to 88. The increase was mostly attributed to a higher rate of new cases among women than among men. The proportion of moderate to severe dementia increased, and mild dementia decreased, mainly because of progression of mild dementias to severer forms and because most new cases were of moderate to severe degree. The proportion of vascular dementia was 47% at age 85 and 54% at 88 despite a higher mortality in vascular than in other dementias. Diagnosis changed to vascular dementia in 9 out of 31 cases of Alzheimer's disease because of new cerebrovascular events. This study illustrates that prevalence is influenced by several factors, such as number of new cases, refusal rate, diagnostic change, and mortality. These factors act in different directions and may differ between populations.