To compare relative frequencies of apolipo-protein E (APOE) alleles in African-American and Caucasian populations and test associations with cognition, we studied two community-based samples: one of 253 African Americans and another of 466 Caucasians age 60-84 years. Logistic regression, adjusting for age, sex, education, and history of hypertension and diabetes was used to associate APOE with five cognitive measures. The APOE-epsilon4 allele frequency was 29.5% in African Americans and 12.1% in Caucasians. In the African Americans, no association was found between the presence of the APOE-epsilon4 allele and any of the cognitive measures. Among Caucasians, APOE-epsilon4 carriers performed more poorly on three of the five tests. We also report a considerably higher frequency of the APOE-epsilon4 allele in our African-American sample compared to other US-based studies.