Previous studies have shown an activation of the hypothalamo-neurohypophyseal system (HNS) in normal aging and in senile dementia. Among other explanations, this activation might be secondary to cell loss in the supraoptic (SO) and paraventricular (PV) nuclei. This study reports a 63% loss in the SO and a 56% loss in the PV in a group of Alzheimer disease (AD) patients. The remaining neurons undergo a compensatory hypertrophy that is more pronounced in the SO, affecting cell and nuclear size as well as nucleolar volume. The group of patients with a diagnosis of moderate dementia showed the greatest hypertrophy, as compared to the severely demented patients. Our results suggest that there is a compensatory capacity in the earlier stages of the dementia, that is lost in the final stages of Alzheimer's disease.