Parkinson's disease (PD) is the second most common neurodegenerative disorder after Alzheimer's disease (AD). Dementia is a common problem late in the course of the disease and there is no effective therapy. Dementia severely impairs patients'' functional status and limits the treatment of the motor manifestations of PD. No effective therapy for dementia in PD is available. The pathophysiology of dementia in PD is not completely understood, however, as in AD, it is thought to be related to cholinergic dysfunction.