The purpose of this research study is to determine whether istradefylline improves cognition in individuals with Parkinson disease with cognitive impairment.

Istradefylline has been approved by the U. S. Food and Drug Administration (FDA) to reduce "off" episodes in Parkinson disease. The period when levodopa has a positive effect on Parkinson's symptoms is called on-time. Once the medication stops working, a so called "off" episode starts, where symptoms recur. Usual care for treatment of Parkinson disease with cognitive impairment is use of cognition enhancing medications also called cholinesterase inhibitors. In this study, participants will receive usual care, and in addition, they will be asked to take istradefylline daily for 26 weeks.

Inclusion Criteria:

• Meet criteria for probable Parkinson disease dementia or PD-MCI (mild cognitive impairment)
• Age greater than 50
• Hoehn and Yahr stage < 4 in "on" state
• Currently taking carbidopa/levodopa
• Antiparkinsonian medications stable for at least 4 weeks prior to baseline visit
• Cholinesterase inhibitor dose stable for 8 weeks prior to baseline visit

Exclusion Criteria:

• Meet criteria for dementia with Lewy bodies, including dementia onset prior to or within 1 year of parkinsonism onset
• Presence of troublesome dyskinesias
• Pregnancy or possibility of becoming pregnant during the study period.
• Moderate or severe hepatic impairment
• dementia too severe to complete study measures or to adhere to medication schedule