In a white, predominantly middle class southern California retirement community, 817 adults aged 65-99 years were evaluated in 1980-1982 in a study designed to assess prevalence of early Alzheimer-type senile dementia. Based on 162 neurologically diagnosed cases, the age- and sex-adjusted prevalence of Alzheimer's disease in the target population was 15.3% (95% confidence interval = 13.0-17.5) among persons aged 65+ years and 35.8% among persons aged 80+ years. Age-adjusted correction for false negatives not detected by a screening procedure yields an estimated prevalence of 16.4% in persons aged 65+ years. Validity of the diagnostic approach was confirmed by a follow-up study. These rates far exceed past prevalence estimates for this disease. Since prevalence of mid- or later stages of Alzheimer's disease may have been underestimated, the high rates in this study may reflect a considerable community burden of early disease. Observed prevalence of Alzheimer-type senile dementia was higher among men than women after adjustment for age, 19.1% and 13.1%, respectively. The female predominance reported in many past clinical surveys may be peculiar to late disease and reflect care-seeking and care-utilization behaviors, rather than differences in incidence.