Concept formation was compared in 71 parkinsonian patients and 35 controls matched for age and WAIS vebal IQ. It was found that the patients formed significantly fewer concepts, as measured by the Wisconsin Card Sorting Test (WCST). The patients had difficulties in shifting sets and were unable to attend to the completion of a concept. Levodopa-treated patients were able to produce more correct responses, but were still unable to maintain the correct responses to produce more correct concepts than non-levodopa-treated patients. The behavior of the parkinsonian patients on the WCST is similar to that of patients who have undergone frontal lobectomy. In addition, the results indicate that levodopa therapy may improve "vigilance", without increasing the patient's overall cognitive ability.