Parahippocampal gyrus was sampled from human brains at autopsy to form three groups: adult (n = 5, mean age 51.2 years), normal aged (n = 5, mean age 79.6), and senile dementia (SD) (n = 5, mean age 76.0). Classification as normal aged or senile demented was based on both behavioral and neuropathological criteria. Tissue was processed for Golgi-Cox, cresyl violet, hematoxylin and eosin and Bodian silver stains. Both atrophied and normal dendritic trees were seen in all cases. Dendrites of layer II pyramidal neurons were quantified with a computer-microscope system. Quantitative data showed that normal aged individuals had longer and more branched dendrites than either adult or SD individuals. There was a slight tendency for SD individuals to have shorter, less-branched dendrites than adults. Differences among groups were greater in apical than in basal portions of the dendritic tree. These differences were largely accounted for by the lengthening and branching (apical dendrites) or lengthening only (basal dendrites) of terminal dendritic segments. These data suggest a model in which aging cortex contains both regressing, dying neurons and surviving, growing neurons. In normal aging it is the latter group that predominates. This is the first demonstration of plasticity in the adult human brain.