The number of dendritic spines on the apical dendrites of layer III pyramidal cells was counted in cerebral biopsies, processed in part according to the rapid Golgi method, which had been performed for diagnostic purposes on patients suffering from dementia of different origins. The samples were from cases affected by Alzheimer's disease (3 patients), Parkinson's disease and dementia (1 patient), Creutzfeldt-Jacob disease (4 patients), Pick's disease (1 patient) and dementia paralytica (1 patient). Spines were counted on consecutive segments of the apical dendrite along the 500-microns proximal region from the cell body. A significant decrease in the number of spines was observed in almost every segment of the apical dendrite in demented patients when compared with similar measurements carried out in age-matched controls (p less than 0.01-0.001); Mann-Whitney U-test. Decreased numbers of dendritic spines may result in reduced inputs on cortical neurons and may be a consistent morphological substrate for the impaired mental status in cortical dementia.