The proposed study will determine whether a therapeutic intervention with donepezil, an acetylcholinesterase inhibitor, shown to be effective in AD, will improve cognitive function in PD. The study is planned as a 26-week, randomized, double-blind, placebo-controlled, cross-over study of the efficacy and safety of donepezil in PD with dementia. The study is being conducted at the University of Pennsylvania, Brown University, Northwestern University and NINDS.