Regional neuronal loss in the substantia nigra was studied in relation to extrapyramidal symptoms and dementia in 12 patients with idiopathic Parkinson's disease (PD) and in 18 control subjects. Four areas of the right substantia nigra were investigated at the level of the superior colliculus and caudal red nucleus. In Parkinson's disease, the percentages of neurons, from the medial to the lateral part of the substantia nigra, were reduced to 49%, 31%, 41%, and 25% of the control values. The number of neurons in the lateral part showed a negative correlation with the severity of rigidity and hypokinesia, whereas tremor was less noticeable in patients with few neurons. The degree of dementia of the patients had a significant correlation only with neuronal loss in the medial part of the substantia nigra, suggesting, in view of the topographical organization of the neurons in the substantia nigra, that intact projections to the caudate nucleus and limbic and cortical areas are a prerequisite for normal cognitive functioning and that their dysfunction leads to clinical dementia.