An increased apolipoprotein E (ApoE) type epsilon 4 allele frequency is associated with both sporadic and familial late-onset Alzheimer's disease (AD). The age of onset of disease in patients homozygous for the epsilon 4 allele appears to be decreased by approximately 15 years compared with E2/3 individuals. In order to assess the influence of this allele on both dementia and cognitive decline in the elderly we have determined the ApoE genotype of 150 individuals over the age of 75 years who have taken part in a longitudinal study. Homozygosity for the epsilon 4 allele was rare. Of the 2 homozygotes, 1 was severely demented but the other did not receive a clinical diagnosis of dementia. The latter individual did demonstrate marked cognitive decline over a 28-month period. There was a consistent association between the presence of an epsilon 4 allele and both the clinical diagnosis of dementia and cognitive decline. These findings confirm a genetic heterogeneity in late-onset sporadic AD and prompt caution in the use of ApoE genotype to predict an elderly individual's susceptibility to either dementia or cognitive decline.