This study sought to disambiguate the impact of Parkinson's disease (PD) on cognitive control as indexed by task set switching, by addressing discrepancies in the literature pertaining to disease severity and paradigm heterogeneity. A task set is governed by a rule that determines how relevant stimuli (stimulus set) map onto specific responses (response set). Task set switching may entail reconfiguration in either or both of these components. Although previous studies have shown that PD patients are impaired at switching between stimuli, in the present study not all patients were impaired at switching entire task sets, that is, both stimulus and response sets: compared with controls, patients with unilateral signs (Hoehn & Yahr Stage I) demonstrated intact switching, even following withdrawal from dopaminergic medication, while bilaterally affected Stage II patients were impaired. The parametric measure of Unified Parkinson's Disease Rating Scale (UPDRS) score predicted increasing switch costs within the patient group. These findings suggest that switching entire task sets may be a function of extrastriatal, possibly non-dopaminergic pathology which increases as the disease progresses. (Contains 3 figures and 6 tables.)