Studies of Asian populations generally have reported prevalence rates for dementia similar to those of predominantly Caucasian populations, but relative prevalence rates of Alzheimer's disease and vascular dementia have differed. Between May 1, 1992 and May 1, 1994, the prevalence rates of dementia, Alzheimer's disease, and vascular dementia were examined in the Japanese American population aged over 65 years in King County, Washington State. A total of 3,045 eligible individuals were identified in a census of persons who were of at least 50% Japanese heritage. Of 1,985 persons who participated in the baseline examination, 382 individuals of 450 sampled from all cognitive performance strata received a diagnostic evaluation. A total of 107 cases with a Clinical Dementia Rating (CDR) of > or = 1 met criteria for dementia according to the Diagnostic and Statistical Manual, 3rd edition, revised (DSM-III-R); 58 of these cases were diagnosed with Alzheimer's disease and 24 with multi-infarct dementia. The estimated prevalence rate for all dementias was 6.3% (95% confidence interval 5.9-6.8). Prevalence rates for dementia increased continuously with age and were 30%, 50%, and 74% for participants aged 85-89, 90-94, and > or = 95 years, respectively; for Alzheimer's disease, prevalence rates were 14%, 36%, and 58% for these three age groups. Rates for Alzheimer's disease were generally higher among women; for multi-infarct dementia, rates for men and women were similar. In the institutional population, the prevalence rate was 66%, and in the community, 2.9%. Persons with lower education had higher overall rates of dementia than those with higher education, but this tendency became weak and inconsistent when rates were age-stratified. The prevalence of dementia in this geographically defined population of Japanese Americans was somewhat higher than prevalence rates reported from Japan, and the distribution of dementia subtypes more closely resembled that found in Caucasian populations in North America and Europe than previously reported in Asian populations.