Brain nicotinic receptors were studied in the frontal cortex, temporal cortex, hippocampus and caudate nucleus in patients with Parkinson's disease (PD), Alzheimer's disease (AD) and control. The Bmax and Kd values of (-)-[3H]nicotine binding were determined with a Scatchard analysis. The number of nicotinic receptors declined both in PD and in AD patients in all brain areas examined. The Kd values were unchanged. There was a negative correlation between the degree of dementia in PD patients and the number of nicotinic receptors in the frontal cortex. A similar correlation was seen between the muscarinic/nicotinic receptor ratio in the frontal cortex and the degree of dementia in PD patients. The present findings indicate that nicotinic receptors are affected not only in AD, but also in PD and that dysfunction of the cholinergic system in the frontal cortex is involved in the dementia process in PD.