The persistent effects of unilateral deep brain stimulation (DBS) of the globus pallidus interna (GPi) or subthalamic nucleus (STN) on specific movement parameters produced by Parkinson's disease (PD) patients are poorly understood. The aim of this study was to determine the effects of unilateral GPi and STN DBS on the force-producing capabilities of PD patients during maximal efforts and functional bimanual dexterity. Clinical and biomechanical data were collected from 14 unilaterally implanted patients (GPi=7; STN=7), at least 13 months post-DBS surgery, during On and Off stimulation in the absence of medication. Unilateral DBS of either location produced a 33% improvement in UPDRS motor scores. Significant gains in maximum force production were present in both limbs during unimanual efforts. The greatest increase in maximum force, for both limbs, was under bimanual conditions. Force in the contralateral limb increased more than 30% during bimanual efforts while ipsilateral force increased by 25%. Unilateral DBS improved grasping force control and consistency of digit placement during the performance of a bimanual dexterity task. The clinical and biomechanical data indicate that unilateral DBS of GPi or STN results in persistent improvements in the control and coordination of grasping forces during maximal efforts and functional dexterous actions. Unilateral DBS implantation of either site should be considered an option for those patients in which bilateral procedures are contraindicated.