As part of a community-based study of Alzheimer's disease (AD) in the African-American population age 65 and over, we have determined apolipoprotein E (Apo E) genotypes in 85 subjects (31 AD patients and 54 controls). The epsilon 4 allele of Apo E was strongly associated with AD in this population sample. The epsilon 4 allele frequency in AD patients was 40.3% compared with 13.9% in the control group, and 22.6% of the AD patients were homozygous for this allele compared with 3.7% of the control subjects (p = 0.01). This study extends the association of Apo E-epsilon 4 and AD to nonwhite populations and provides further evidence that the observed allelic association is biologically relevant.