According to their mental status, patients with Parkinson's disease can be subdivided into three groups: (1) mentally normal patients; (2) patients with severe cognitive impairment and Alzheimer-type brain pathology (neuritic plaques, neurofibrillary tangles, granulovacuolar changes); and (3) demented patients without any evidence of Alzheimer changes. Neurochemically, irrespective of the presence or absence of Alzheimer-type brain pathology, demented Parkinson patients seem to have the same disturbance of cortical cholinergic neuron function as patients with Alzheimer-type dementia (Alzheimer's disease), namely, reduced levels of cortical acetylcholine esterase and choline acetyltransferase activity. At present, the question whether the "cortical cholinergic deficiency" is the only (or sufficient) neurochemical basis for the cognitive impairment in Parkinson patients with dementia cannot be answered with certainty; the additional role of other neurotransmitter changes known to occur in the Parkinson brain, especially loss of cortical, hippocampal and subcortical noradrenaline and/or dopamine cannot be ruled out.