The purpose of this investigation was to document the prevalence of severe dementia among different racial groups residing in the same community--Copiah County, Mississippi. Both household and institutionalized populations were included. For either sex, the prevalence ratios of all severe dementia and clinically diagnosed severe senile dementia of the Alzheimer's type were at least as large among blacks as among whites. For either race, the corresponding prevalence ratios were greater in females. For each race and sex, the corresponding prevalence ratios increased with advancing age. Finally, in the population studied, approximately 1% of individuals 40 years old or older had severe dementia. This figure increased to 7% for individuals 80 years old or older.