Hyperactivity in the subthalamic nucleus (STN) is seen in animal models of Parkinson's disease, and lesioning of the STN dramatically relieves the animal's parkinsonism. Deep brain stimulation (DBS) of the STN is an effective treatment for patients with advanced Parkinson's disease. We have studied the effects of a unilateral lesion placed in the STN in predominantly hemi-parkinsonian patients. Twenty-one patients with advanced idiopathic Parkinson's disease were studied. Seventeen had asymmetrical tremor-dominant Parkinson's disease and four had bilateral disease. All patients underwent radiofrequency lesioning of the dorsolateral part of the STN under stereotactic guidance. The four patients with bilateral disease had, in addition, an electrode implanted contralaterally in the STN. Twenty-one patients have been followed for a minimum of 12 months. Clinical evaluation included the use of the Unified Parkinson's Disease Rating Scale (UPDRS) before and after surgery. Post-operative high-resolution MRI was performed in each patient to confirm lesion location, and this was correlated with clinical outcome. There was improvement in contralateral tremor, rigidity and bradykinesia in all patients followed for 6, 12 and 24 months, with the effect on tremor being greatest. L-dopa equivalent daily intake was approximately halved, and this resulted in a significant reduction in dyskinesia. Psychometric test scores were mostly unchanged or improved. All lesions were successfully located in the dorsolateral STN. Nineteen of the 21 lesions extended beyond the STN to involve pallidofugal fibres (H2 field of Forel) and the zona incerta (ZI). Lesion-induced dyskinesias were not a management problem except in one patient, whose lesion was confined to the STN. This patient was successfully treated with deep brain stimulator placement in the region of H2/ZI. Unilateral STN lesions can be made safely and are an effective alternative to thalamotomy, pallidotomy and unilateral STN DBS for the treatment of asymmetrical tremor-dominant advanced Parkinson's disease. Com bined lesioning of the dorsolateral STN and H2/ZI is particularly effective.