Over the past decade, deep brain stimulation (DBS) has become an effective treatment option for managing severe Parkinson's disease (PD). However, evidence is accumulating that DBS of target sites like the subthalamic nucleus (STN) can result in unintended cognitive effects that lie beyond motor control. The aim of the present study was to evaluate whether changes in executive task performance after chronic DBS might be predominantly associated with the stimulation of only one hemisphere. Eight patients with PD who had undergone DBS treatment of the STN were selected to participate in the study. Using a repeated measurements design, they underwent a neuropsychological examination under unilateral left- and right-sided stimulation in order to investigate laterality effects in their performance on the Wisconsin Card Sorting Test. All patients showed a significant improvement in motor symptoms postoperatively. Selected aspects of executive task performance were compromised under left- when compared to right-sided stimulation. Performance measures were unrelated to demographic, neurological, and behavioral characteristics of the patients. Findings are consistent with the emerging evidence that the STN is not only involved in motor control, but also participates in functions of the cognitive domain. Moreover, results raise the possibility that the left and right hemisphere might differ in their vulnerability to tolerate side effects on executive functions of DBS treatment. Potential consequences for future research questions and the management of cognitive side effects are discussed.