Neuropsychological tests known to reveal abnormalities in patients with frontal lobe damage were used to explore cognitive function in 30 mildly disabled, right-handed patients with idiopathic Parkinson's disease. None of the patients had received treatment and all had normal CT brain scans. Patients with depression or high ischaemia scores were excluded from the study and similar selection criteria were used for the age-matched controls. No impairment of general intellectual function was found in the patients using the WAIS and New Adult Reading IQ tests and no abnormalities were apparent on cognitive estimates and two-choice Recognition Memory Tests. Patients with Parkinson's disease, however, had significantly greater difficulty in shifting conceptual sets and produced more perseverative errors on both the modified Wisconsin Card Sorting Test and Benton's Word Fluency Test. These subtle cognitive difficulties might underlie the mental inflexibility and rigidity of Parkinson's disease and could be attributed to destruction of the ascending dopaminergic mesocorticolimbic pathway.