Findings from a Golgi study of the visual cortex in patients with the Down syndrome were compared with those from neurologically normal, age-matched control subjects. The dendritic atrophy seen in childhood continued into adulthood, with a marked decrease in dendritic branching, dendritic length, and spine frequency in elderly adults with the Down syndrome. Subject more than 30 years old occasionally had degenerating pyramidal neurons in the cerebral cortex and degenerated pyramidal neurons and aspiny stellate cells, particularly in the temporal cortex. These dendritic abnormalities may be related to mental retardation in children and early dementia in older adults who have the Down syndrome. The genetic and extrinsic factors may be important determinants of Alzheimer type dementia in the Down syndrome.