OBJECTIVE: To estimate the prevalence of dementia and its subtypes by sex and age group for five regions of Canada. DESIGN: Prevalence survey. SETTING: Community and institutional settings in Canada, excluding those in the two territories, Indian reserves and military units. PARTICIPANTS: Representative sample of people aged 65 and over interviewed between February 1991 and May 1992. Those in the community (9008 subjects) were chosen randomly from medicare lists in nine provinces or from the Enumeration Composite Record in Ontario. People in institutions (1255) were randomly selected from residents in stratified random samples of institutions in each region. INTERVENTIONS: Screening with the Modified Mini-Mental State (3MS) Examination to identify cognitive impairment. Clinical examination of all those in institutions, those in the community with a 3MS score of less than 78 and a sample of those in the community with a 3MS score of 78 or more to diagnose dementia. Dementia and Alzheimer's disease were defined according to established criteria. MAIN OUTCOME MEASURES: Prevalence of dementia of all types, by region, sex and age group, the estimated number of cases in the population by type of dementia and the age-standardized rate per 1000 population. RESULTS: The prevalence estimates suggested that 252,600 (8.0%) of all Canadians aged 65 and over met the criteria for dementia (95% confidence interval [CI] 236,800 to 268,400). These were divided roughly equally between the community and institutional samples; the female:male ratio was 2:1. The age-standardized rate ranged from 2.4%, among those aged 65 to 74 years, to 34.5%, among those aged 85 and over. The corresponding figures for Alzheimer's disease were 5.1% overall (161,000 cases; 95% CI 148,100 to 173,900), ranging from 1.0% to 26.0%; for vascular dementia it was 1.5% overall, ranging from 0.6% to 4.8%. If the prevalence estimates remain constant, the number of Canadians with dementia will rise to 592,000 by 2021. CONCLUSIONS: These Canadian estimates of the prevalence of dementia fall toward the upper end of the ranges in other studies, whereas the estimates for Alzheimer's disease fall in the middle of the ranges. This may suggest an unusual balance between Alzheimer's and other forms of dementia in the Canadian population.