Twenty consecutive patients with idiopathic Parkinson's disease underwent stereotactic posteroventral pallidotomy. Schwab and England ADL scores in the "off" state were improved by 18% and in the "on" state the scores declined by 2%. Three patients also reported marked improvement in "off" state dystonia. One-year data are available on 12 patients who underwent evaluations according to the Core Assessment Program for Intracerebral Transplantation protocol preoperatively and at 3, 6, and 12 months after surgery. Significant improvements in Unified Parkinson's Disease Rating Scale sections II and III scores in the "off" state, composite "off" state scores of bradykinesia and rigidity, contralateral tremor in the "off" state, and contralateral dyskinesias were observed. Although there was reduction in the daily levodopa dose, this did not reach statistical significance. Major complications (15%) included hemiparesis (one of 20) and visual field cuts (two of 20); minor complications (45%) included mild cognitive dysfunction (four of 20), reading difficulty not related to visual disturbance (one of 20), and 5-10 lb weight gain (four of 20).