Senile plaques (SP) are one of the major neuropathologic hallmarks of senile dementia of Alzheimer type (SDAT). The regional distribution of SP stained for acetylcholinesterase (AChE) has been quantified in the hippocampal formation of patients with SDAT. Consistent differences in plaque distribution and density were observed between different patients. SP were significantly more numerous in the outer dentate molecular layer, whereas their density was low in other hippocampal regions. The quantity of AChE positive SP seemed to correlate with the density of neurofibrillary tangles in the entorhinal cortex. The AChE positive SP had a pattern of distribution different from the one observed with Thioflavin S. These results are discussed in light of a possible sprouting of cholinergic septal afferents.