We report the safety results in nine patients with advanced idiopathic Parkinson's disease (PD) who underwent ablative surgery of unilateral subthalamic nucleus (STN). In eight patients, surgical objectives were attained without induction of abnormal involuntary movements or other adverse effects. One patient developed transient hemiballistic movements which improved within 2 weeks after surgery. Assessment at 2 weeks to 20 months postoperatively revealed no long-term adverse effects. We conclude that hemiballism following unilateral ablation of STN in patients with PD is a rare phenomenon, and unilateral ablative lesions of STN can be performed safely. © 2001 Movement Disorder Society.