Summary:The simultaneous influence of age at onset of dementia, duration of illness, and their interaction on neuropsychological functioning was examined in 124 patients with probable Alzheimer's disease. Three cognitive factors (reflecting spatial processing, naming ability, and verbal fluency and comprehension) were derived by principal components analysis. Multiple regression analyses revealed that early onset of illness is associated with more profound deficits in all three cognitive domains. More importantly, they indicated that the effect of disease chronicity (i.e., duration of illness) on particular language tasks depends on the age at which symptoms begin. Therefore, future studies of age at onset as a subtyping variable in Alzheimer's disease should consider the influence of duration of illness, and vice versa.