The neuronal number and nucleolar volume of the magnocellular population of the basal nucleus of Meynert in 6 patients with progressive supranuclear palsy (PSP) - one associated with Alzheimer's disease - were compared with those of 5 controls. In the 5 typical PSP cases, an age-dependent neuronal loss was demonstrated ranging from 12.6% to 54.1%. In the PSP-Alzheimer patient, the loss of cells amounted to 73.8% and the surviving neurons exhibited a reduction in the nucleolar volume, suggesting a reduced protein synthetic capability. The involvement of the basal nucleus is therefore a usual anatomical feature of PSP. The damage is less severe than generally observed in Alzheimer's disease and biochemical studies are required to state whether this is accompanied by a substantial cholinergic deficit.