It has been suggested that patients with Parkinson's disease have a basic deficit in shifting "set." Previous authors have used this hypothesis as an explanation for deficits on a range of cognitive tasks. An experiment was conducted to investigate this phenomenon. A reaction time paradigm was used in which the subject had to make left-right decisions under two stimulus conditions. Both patients and controls showed a facilitation in performance with repetition within each block of trials, followed by a deterioration in performance when the stimulus type switched. Furthermore, the shapes of the reaction-time functions were equivalent in both groups, suggesting that the patients had no deficit in either switching or establishing set in this task. In contrast, they were impaired on the Wisconsin Card Sorting Test. This dissociation suggests that patients with Parkinson's disease do not have a generalized deficit in switching set. It is suggested that patients with Parkinson's disease are impaired on tasks where they have to rely upon internal control for the regulation of behaviour, but are relatively unimpaired on tasks where external cues are available.