Advanced Parkinson's disease (PD) is frequently associated with dementia. The pathogenesis of this dementia is complex, related to deficiency of several biogenic amines and cortical Lewy body deposition, as well as co-existent age-related brain changes, both of the Alzheimer type and vascular. However, degeneration of the cholinergic neurons in the nucleus basalis of Meynert may have an important contribution to the cognitive decline. The dementia of PD has a grave effect on the quality of life of the patients and their caregivers, as well as a negative effect on their survival. The treatment of dementia associated with PD with cholinesterase inhibitors produced gratifying (although limited) results. Future studies should define the exact role of these agents in the treatment of the dementia of PD. Another major problem presented by demented PD patients is the occurrence of delusions and hallucinations, which make the life of patients and caregivers miserable. Classical neuroleptics are of course contra-indicated in these patients but recent data increase concern about the safety of novel derivatives, leaving a void in the pharmacological armamentarium available when these manifestations appear.