The authors classified 100 patients with Alzheimer disease (AD) as presenting with a memory or nonmemory phenotype. APOE genotype was determined. There was an association between APOE-epsilon4 and clinical phenotype (odds ratio = 3.0; 95% CI: 1.2 to 7.8), suggesting that two subtypes of AD can be identified. The typical amnestic phenotype seems to be promoted by the APOE-epsilon4 allele, whereas the atypical nonmemory phenotype occurs in the absence of the APOE-epsilon4 allele.