To identify the focus of impairment in the performance of sequential movements of patients with Parkinson's disease, the extent of their reliance on external cues was examined. Eighteen patients with idiopathic Parkinson's disease and their matched controls performed a series of button presses at sequential choice points along a response board. The illuminated pathway to be followed successively extinguished ahead of each move according to three levels of reduction of external cues. Patients with Parkinson's disease were particularly disadvantaged with high levels of reduction of external cueing in terms both of movement preparation time (button down time) and movement execution time (movement time between buttons). Moreover, with high levels of reduction of external cueing, patients with Parkinson's disease were particularly subject to progressive slowing (movement time, not down time) further down the sequence. The basal ganglia may help generate internal cues for releasing successive stages of a predefined movement sequence.