['To determine whether APOE epsilon 4 predicts rate of cognitive change in incident and prevalent Alzheimer disease (AD).', 'Individuals were recruited from two longitudinal cohort studies-the Washington Heights and Inwood Columbia Aging Project (WHICAP; population-based) and the Predictors Study (clinic-based)--and were followed for an average of 4 years. Three samples of participants diagnosed with AD, with diverse demographic characteristics and baseline cognitive functioning, were studied: 1) 199 (48%) of the incident WHICAP cases; 2) 215 (54%) of the prevalent WHICAP cases; and 3) 156 (71%) of the individuals diagnosed with AD in the Predictors Study. Generalized estimating equations were used to test whether rate of cognitive change, measured using a composite cognitive score in WHICAP and the Mini-Mental State Examination in Predictors, varied as a function of epsilon 4 status in each sample.', 'The presence of at least one epsilon 4 allele was associated with faster cognitive decline in the incident population-based AD group (p = 0.01). Parallel results were produced for the two prevalent dementia samples only when adjusting for disease severity or excluding the most impaired participants from the analyses.'] Does aPOE epsilon 4 allele predict faster cognitive decline in mild Alzheimer disease?