The outcome of radiofrequency-guided posteroventral medial pallidotomy was investigated in 29 patients with recalcitrant Parkinson's disease. Extracellular recordings were obtained in the target region to differentiate the internal from the external globus pallidus, and distinct waveforms were recorded in each region. Stimulation of the target site further verified the lesion location. Of the 29 patients treated during the course of 1 year, none showed any adverse side effects (such as hemianopsia or hemiparesis) from the procedure. Significant and immediate improvement in motor involvement (dyskinesia, rigidity, dystonia, freezing, and tremor) was observed as measured by the Unified Parkinson's Disease Rating Scale and the Hoehn and Yahr scale. Patients experienced improvements in their condition as measured on a self-rating scale, and their ability to perform the activities of daily living was also significantly improved. Although the onset and duration of the effect of a single dose of levodopa did not change, the number of hours in an "off" state of dyskinesia per day was significantly decreased. These results provide further evidence, in a large group of patients, that posteroventral medial pallidotomy results in significant control of the motor symptoms of Parkinson's disease with a minimum of undesirable side effects.