Neurophysiological and behavioral data obtained from 9 patients with presenile dementia and 10 with senile dementia of the Alzheimer type were compared with similar data from 25 age- and sex-equivalent controls. Compared with the healthy controls, both patients groups demonstrated increased background electroencephalographic slowing with a reduction in fast activity (synchronization). Topographic analyses of data from electroencephalographic and evoked potential studies indicate that areas of maximal group differences between the presenile patients and their controls include the right posterior temporal and, to a lesser extent, left midtemporal to anterior temporal areas, whereas the maximal differences between the senile patient group and their controls involve the midfrontal and anterior frontal lobes, bilaterally. Moreover, right-sided numerical features derived from topographic maps proved most useful in differentiating the presenile patients and their age-matched controls, whereas bilateral features were more useful in separating senile patients from their controls. These topographic dissimilarities between patient groups suggest that an age-disease interaction exists between patients with presenile and senile dementia of the Alzheimer type. Correlational analyses between neuropsychological test scores and neurophysiological features indicate that increased slowing and decreased fast activity were associated with poorer test performance.