A prospective, randomized, double-blind pilot study to compare the results of stereotactic unilateral pallidotomy and subthalamotomy in advanced idiopathic Parkinson's disease (PD) refractory to medical treatment was designed. Ten consecutive patients (mean age, 58.46.8 years; 7 men, 3 women) with similar characteristics at the duration of disease (mean disease time, 8.43.5 years), disabling motor fluctuations (Hoehn Yahr stage 3-5 in off-drug phases) and levodopa-induced dyskinesias were selected. All patients had bilateral symptoms and their levodopa equivalent dosing were analysed. Six patients were operated on in the globus pallidus interna (GPi) and four in the subthalamic nucleus (STN). Clinical evaluation included the use of the Unified Parkinson's Disease Rating Scale (UPDRS), HoehnYahr score and Schwab England activities of daily living (ADL) score in 'on'- and 'off'-drug conditions before surgery and 6 months after surgery. There was statistically significant improvement in all contralateral major parkinsonian motor signs in all patients followed for 6 months. Levodopa equivalent daily intake was significantly reduced in the STN group. Changes in UPDRS, Hoehn Yahr and Schwab England ADL scores were similar in both groups. Cognitive functions were unchanged in both groups. Complications were observed in two patients: one had a left homonymous hemianopsia after pallidotomy and another one developed left hemiballistic movements 3 days after subthalamotomy which partly improved within 1 month with Valproate 1000 mg/day. The findings of this study suggest that lesions of the unilateral STN and GPi are equally effective treatment for patients with advanced PD refractory to medical treatment.