Samples of the hippocampus of four patients with Down's syndrome [two men aged 35 and 36 years with no evidence of Alzheimer's disease (AD) and two patients aged 47 and 55 years with associated AD] were obtained at post mortem and processed according to the rapid Golgi method. A significant reduction in the number of dendritic spines (DS) was found in the apical (middle, distal and oblique segments) and basilar (thick and thin segments) dendritic arbors of CA1 and CA2-3 pyramidal neurons in patients with Down's syndrome and no AD when compared to age-matched controls. An additional decrease of DS in every segment occurred in Down's patients with associated AD when compared to age-matched controls and Down's patients with no AD. In Down's syndrome (either associated or not to AD) thin basilar dendrites were the most severely involved; in AD patients CA1 pyramids were more severely affected than pyramidal neurons of the CA2-3 subfield.