The recent finding of neuronal degeneration in the nucleus basalis of Meynert (nbM) in Alzheimer's disease suggests a possible role of this nucleus and the cholinergic system in dementing illness. We assessed neuronal population and density in two anatomical levels of the nbM in 38 coded brains from patients with a broad variety of neurological disorders. We found the most striking nbM degeneration in Alzheimer's and Parkinson's diseases. An intermediate loss of neurons was noted in the transmissible dementia Creutzfeldt-Jakob disease and in progressive supranuclear palsy. The usually nondementing diseases amyotrophic lateral sclerosis and multiple sclerosis showed cell counts comparable to those in a control group of nondemented patients with cerebrovascular disease. Characteristic neuropathological changes of Alzheimer's, Parkinson's, and Creutzfeldt-Jakob diseases, progressive supranuclear palsy, and subacute sclerosing panencephalitis were also found in the nbM. Most previous studies of the nbM have been based on small numbers of cases compared with controls. In our large series we used consistent methodology and a multiple-comparison statistical procedure to avoid overreporting of statistical significance. Our comparison of the nbM not only confirms the marked degeneration of the nbM in Alzheimer's disease, but also places such degeneration into perspective in a spectrum of dementing and nondementing neurological diseases. Larger case studies, using appropriate statistical techniques for multiple group comparisons, are needed to establish the significance of nbM degeneration in neurological disease.