OBJECTIVE: To evaluate the effects of bilateral deep brain stimulation in the subthalamic nucleus for symptomatic relief of advanced idiopathic Parkinson's disease. DESIGN: Prospective cohort study. SETTING: Patients were assessed and received medical treatment at the Kingston Centre, Southern Health, Melbourne. Surgery took place at Melbourne Neuroscience Centre, The Royal Melbourne Hospital. Both are tertiary public institutions. SUBJECTS: 14 patients with Parkinson's disease with intact cognition and difficult to manage motor symptoms who were referred to Kingston Centre between 1996 and 2000 and were eligible for surgical intervention. INTERVENTIONS: All patients were assessed both after 12 hours' withdrawal from and while taking their levodopa medication on two occasions before surgery. Further assessments were carried out one, three, six and 12 months after surgery. MAIN OUTCOME MEASURES: The Unified Parkinson's Disease Rating Scale motor exam and gait parameters, such as stride length and velocity, were compared at six months after surgery with neither stimulation nor medication, with stimulation only, with medication only, and with stimulation and medication. RESULTS: Stimulators were explanted in one patient after intracranial haemorrhage and relocated to the thalamus in a second. Extraneous factors prevented two patients from attending at six-month follow-up. Motor performance improved significantly with stimulation alone in the 10 remaining patients. Further significant gains were seen with stimulation and medication combined, with an apparent reduction in side-effects such as dyskinesia. CONCLUSIONS: Bilateral deep brain stimulation of the subthalamic nucleus significantly improves motor performance in advanced Parkinson's disease, despite a rather high complication rate.