An extensive neuropsychological battery (the Mental Deterioration Battery) was utilized to distinguish, within a sample of 24 idiopathic Parkinsonians, those showing signs of diffuse mental deterioration (n = 9) from those without deterioration (n = 15). Performances of control subjects on a wide range of tests exploring mnesic, visuo-constructive, linguistic and general intellectual functions (n = 21) did not differ from analogous performances of Parkinsonians without signs of diffuse mental deterioration. The Wisconsin Card Sorting Test was then used to verify the hypothesis that a selective impairment of cognitive functions subsumed by the integrity of frontal lobes could be demonstrated in Parkinsonian patients. Our results provide evidence that in this task, defective performances are obtained by Parkinsonians and even by patients without signs of diffuse cognitive impairment. These findings seem to confirm that a deficit in concept formation, maintenance and shifting is largely independent of the dementia frequently noticed in Parkinson's disease.