There is a marked variation in whether people retain sufficient cognitive function to maintain their quality of life and independence in old age, even among those without dementia, so it would be valuable to identify the determinants of normal age-related cognitive change. We have retested non-demented 80-year-olds who were participants in the Scottish Mental Survey of 1932, and find that the variation in their non-pathological cognitive change from age 11 to 80 is related to their apolipoprotein E (APOE) genotype. This effect of the APOE epsilon 4 allele on normal cognitive ageing may be mediated by a mechanism that is at least partly independent of its predisposing effect towards Alzheimer's disease.