Patients with Parkinson's disease (PD) commonly exhibit difficulties performing simultaneous tasks and levodopa has been shown to improve the performance of these movements to a greater extent than movements performed in isolation. The aim of this study was to compare the effects of acute unilateral pallidal lesions (nine patients) and bilateral deep brain stimulation (DBS) of the subthalamic nucleus (STN) (eight patients) with levodopa therapy (ten patients) on the performance of isolated versus bilateral simultaneous repetitive movements. The STN group was assessed with and without DBS both on and off levodopa. The two tasks employed were maximally paced button tapping (Tap) and wrist pronation-supination (WPS) movements. During the off drug state (12-14 h after the last oral dose of levodopa), the performance of simultaneous Tap and WPS movements in all three groups was significantly slower and more irregular than when each movement was performed in isolation. For example, WPS velocity decreased by at least 37% (P<0.05) with concomitant Tap. Following levodopa, pallidotomy or STN DBS, WPS velocity was increased during the simultaneous task to a greater extent than in the isolated task. All treatments also improved WPS velocity and increased the regularity of movement during concomitant Tap (P<0.01). The findings indicate that, like levodopa, surgical therapies can improve the performance of simultaneous tasks more than isolated tasks. These observations suggest that the excessive neuronal activity and/or abnormal firing patterns in the globus pallidus internus that is found in parkinsonian patients contribute to difficulties in the execution of complex motor tasks.