Regional neuronal loss in the substantia nigra (SN) was studied in relation to extrapyramidal symptoms and dementia in 27 patients with Alzheimer's disease (AD), 12 patients with idiopathic Parkinson's disease (PD) and 18 controls. Four areas of the right SN were investigated at the level of the caudal red nucleus. In AD the number of neurons were reduced to 97%, 79%, 83% and 78% of the control values from the medial to lateral SN respectively. The number of lateral neurons showed a negative correlation with the severity of rigidity and hypokinesia. The degree of dementia did not have a significant correlation with nigral neuronal counts. In PD, the number of neurons was markedly reduced in all parts of the SN, most pronouncedly in the lateral part. The number of lateral neurons showed a negative correlation with the severity of rigidity and hypokinesia, whereas tremor had a positive correlation. The degree of dementia of the patients had a negative correlation with the number of medial neurons, suggesting that degeneration of the nigrostriatal neurons may contribute as a subcortical component to dementia in PD patients.