A representative region of the nucleus basalis of Meynert was investigated in 11 patients with idiopathic Parkinson's disease and compared with the identical region in 13 age‐matched control subjects. Simultaneously, the cerebral cortex and the nucleus basalis in the patients with Parkinson's disease were examined for senile plaques and Alzheimer's neurofibrillary tangles. The nucleus basalis was significantly depleted of its large neurons in Parkinson's disease (p < 0.001 versus controls; Student t test), but in the majority of cases the neuron loss was not associated with Alzheimer's disease.