We report on the prevalence of dementia in Canadians age 85 years and older. The purpose of this study was to determine whether the prevalence of dementia continued to increase in the very old, and to define the types of dementia and their relative proportions in this age group. We collected data as part of the Canadian Study of Health and Aging (1990 to 1992), which consisted of a sample of 1,835 subjects from a population of 283,510 Canadians who were 85 years of age and older residing in the community or in institutions. The prevalence of dementia in the 85 years and older group was 28.5%, more than twice that of the 75- to 84-years cohort. The prevalence of dementia of 23% in the 85- to 89-years sample (n= 1,332) increased to 40% in the 90 to 94 years group (n = 371) and, in the 95 years and older sample (n = 104), reached 58%. Overall, Alzheimer's disease (AD; probable or possible) accounted for 75% of all dementias; a vascular etiology alone accounted for 13% of dementias. The proportion of clinically diagnosed AD cases to vascular dementia cases increased significantly after age 65 and was higher in the 85+ group than in a younger cohort (65 to 84 years).