A prospective longitudinal study was undertaken in a geriatric hospital on women over 75 years of age, clinically diagnosed as either intellectually normal or having senile dementia of the Alzheimer type (SDAT) of varying degrees of severity. Mental impairment was assessed prospectively. Fifteen brains from this population were studied to evaluate quantitatively the distribution of senile plaques (SP) in relation to cortical lamination. SP density in four neocortical areas (first temporal gyrus; supramarginal gyrus calcarine area; precentral gyrus) was significantly correlated with the degree of mental impairment. SP distribution in the cortical layers was evaluated by an indirect method and appeared to be fairly constant from one case to another. Significantly higher SP densities were observed in layers II and III of the temporal and occipital samples, while minimal values were noted in layer I. Lower densities of SP were found in layers V and IV of the occipital and temporal lobes. These data suggest a selective vulnerability of some areas of cortical projections in SDAT.