The distribution of senile plaques within the hippocampal formation was examined at autopsy in the brains of 18 patients with Alzheimer's disease, ranging in age from 62 to 89 years. Wax sections were cut at the level of the central part of the cornu ammonis and stained by a silver impregnation method. Plaque surface density was determined in the stratum pyramidale of the presubiculum, subiculum, prosubiculum and CA1-4. Senile plaques were present in all 7 regions of the hippocampal formation. However, the number of senile plaques/mm2 was significantly greater in the subicular complex than in the cornu ammonis. There was no difference in plaque density between the individual regions of the subicular complex or between the 4 CA fields.