We have previously shown that patients with Parkinson's disease (PD) perseverate in their choice of action relative to healthy controls, and that this is affected by dopaminergic medication (Hughes LE, Barker RA, Owen AM, Rowe JB. 2010. Parkinson's disease and healthy aging: Independent and interacting effects on action selection. Hum Brain Mapp. 31:1886-1899). To understand further the neural basis of these phenomena, we used a new task that manipulated the options to repeat responses. Seventeen patients with idiopathic PD were studied both "on" and "off" dopaminergic medication and 18 healthy adults were scanned twice as controls. All subjects performed a right-handed 3-choice button press task, which controlled the availability of repeatable responses. The frequency of choosing to repeat a response (a form of perseveration) in patients was related to dopamine therapy and disease severity as a "U-shaped" function. For repetitive trials, this "U-shaped" relationship was also reflected in the BOLD response in the caudate nuclei and ventrolateral prefrontal cortex. Our results support a U-shaped model of optimized cortico-striatal circuit function and clearly demonstrate that flexibility in response choice is modulated by an interaction of dopamine and disease severity.