We carried out a neuropsychological study on cognitive impairment in 57 subjects affected by idiopathic Parkinson's Disease (PD) and 32 subjects affected by Alzheimer's Disease (AD). First, we found two different subgroups of Parkinsonian patients, the first one with and the second without dementia. We clearly identified these two distinct subclinical entities regardless of mean age, age of onset, duration of treatment; on the contrary, the type of treatment seems to play a specific role in the appearance of dementia in PD, anticholinergics being assumed almost exclusively by demented Parkinsonian patients. Second, we observed two main differences for cognitive impairment between PD with dementia and AD. In fact, cognitive impairment is consistently more evident in Alzheimer patients than in Parkinsonian ones with dementia; in addition, demented Parkinsonians show a pattern of impairment similar to that exhibited by patients affected by frontal lobe lesions. This result supports neuroanatomical and neurochemical data on the involvement of the whole dopaminergic system in PD and the role played by the ventromedial tegmental area projecting to the frontal cortex in causing cognitive dysfunction in this disease.