This is a multicenter, open-label, parallel group, single-dose study to determine the single-dose PK of istradefylline in subjects with mild hepatic impairment (HI) (Child-Pugh Class A) and in subjects with normal hepatic function. Ten subjects with mild HI (Child-Pugh Class A) and 10 subjects with normal hepatic function (matched for age, gender, race, and BMI) will be enrolled. Enrollment of the subjects with normal hepatic function will be subsequent to the HI subjects.