Longitudinal data from the MacArthur Study of Successful Aging were used to test for interactions between education and apolipoprotein E (APOE) genotype with respect to time trends in cognitive performance. Interactions between education, APOE-e4 status, and time were found for overall cognitive function, and for subscales measuring memory and naming: The presence of the e4 allele was associated with steeper declines in cognition for those with a greater than eighth-grade education. For those with an eighth-grade education or less, time trends did not differ by APOE genotype. A measure of cognitive impairment (i.e., scores of < or = 7 on the Short Portable Mental Status Questionnaire) yielded parallel though weaker evidence for a similar interaction with respect to risk of cognitive impairment. These findings suggest that the presence of at least one e4 allele appears to reduce the protective effects of education for those with at least a ninth-grade education or more, resulting in steeper cognitive declines with age.