To determine the prevalence of cognitive impairment and dementia in a multi-ethnic community, we examined a population sample of 2,759 elderly (65 years of age and older) African American, Hispanic-Cuban and white non-Hispanic men and women of Dade County, Florida. The Short Portable Mental Status Questionnaire (SPMSQ) was used as a screening test. The prevalence of cognitive impairment for African American men was 17.0% and women 16.7%; Cuban men 9.4% and women 11.4%; and white non-Hispanic men 9.0% and women 8.5%. Participants with cognitive impairment were referred to two Memory Disorder Clinics for diagnosis of dementia/Alzheimer's disease (AD). SPMSQ cutpoints took account of race and education. The prevalence of dementia/AD was adjusted for sensitivity and specificity of the SPMSQ in each sex/ethnic group. The prevalence of dementia among African American men (20.9%) was twice that among white non-Hispanic men (11.6%). White non-Hispanic and Cuban women had a similar prevalence of dementia (12.1% vs. 12.9%). Low SPMSQ specificity for Cuban men and African American women gave unstable dementia prevalence estimates. More than two thirds of all dementia cases had AD, and among white non-Hispanics, women had double the prevalence of AD among men (10.9% vs. 5.4%). The prevalence of AD among African American men was more than two and a half times greater than the prevalence among white non-Hispanic men (14.4% vs. 5.4%). Age (p = 0.001), family history of AD (p = 0.02) and African American (p = 0.0001) or Cuban (p = 0.006) ethnic group were directly and independently associated with the prevalence of AD.