Diffuse serotonergic fibers are presumed to project to the telencephalon from the nucleus raphe dorsalis (NRD) of the midbrain, in a manner similar to the cholinergic projections from the nucleus basalis of Meynert (nbM) to the cerebral cortex. Neuropathological changes in both of these nuclei have been reported in Alzheimer's disease (AD). Although many morphometric studies of the nbM in AD have been documented, only one such study of the NRD has been conducted so far; it demonstrated a sixfold increase in neurofibrillary tangles in AD but no statistically significant difference in the number of neurons in patients with AD and age-matched controls. A study of the NRD utilizing different stains and wider anatomical boundaries is detailed in this report of 5 patients with AD and 7 age-matched controls. In AD the NRD showed 39 times more neurofibrillary tangles and the number and cell density of large neurons were reduced to 23% and 28%, respectively, of those in the controls. A small number of senile plaques were found in the NRD in all patients with AD but none were found in the controls.