In this study we established that cognitive shifting, an ability that is known to be affected in PD, is more impaired in PD patients, treated with anticholinergics, than in de novo patients. Eleven PD patients on anticholinergic monotherapy were compared with 30 de novo patients. The groups did not differ with respect to age, duration and severity of PD, and depression, nor with respect to general intelligence or attention. We assessed cognitive shifting with three different card-sorting tests. The patients on anticholinergics showed a poorer performance on all card-sorting tests than the de novo patients did. The patients on anticholinergics needed significantly more trials in two card-sorting tests and discovered significantly less categories in total. There was also a significant difference in memory performance, but memory performance did not correlate with any score on the card-sorting tests. This indicates that the performance on card-sorting tests and the memory performance were independent.