Objective: To evaluate the long term (4 years) efficacy of deep brain stimulat ion (DBS) of the subthalamic nucleus (STN) in advanced Parkinsons disease. Met hods: We performed a double blind crossover evaluation of the efficacy of DBS of the STN in the“off”medication condition in 10 patients with Parkinsons dise ase. Assessments included the Unified Parkinsons Disease Rating Scale (UPDRS) part III (motor) and two timed tests (arm tapping and walking). Open evaluation of the effect of stimulation in the off and on drug states preoperatively and at 1 and 4 years postoperatively was also conducted. The latter assessment include d the UPDRS parts II (activities of daily living) and III (dyskinesia scale and global assessment) as judged by the patient and examiner. The mean amount of lev odopa daily dose at base line, 1 year, and 4 years after surgery was compared. R esults: A significant (p < 0.04) effect of stimulation was observed in the overa ll group regarding both the UPDRS motor and the timed tests. Open evaluation als o showed a significant benefit of STN DBS with respect to preoperative assessmen t in both the motor and activities of daily living scales, dyskinesia scale, and in global assessment. Levodopa daily dose was reduced by 48%and 50%at 1 and 4 years, respectively. There was no difference between the 1 and 4 years evaluati ons in any of the parameters evaluated. Complications due to stimulation were mi nor. Conclusions: DBS of the STN provides a significant and persistent anti par kinsonian effect in advanced Parkinsons disease 4 years after surgery. Objective: To evaluate the long term (4 years) efficacy of deep brain stimulation (DBS) of the subthalamic nucleus (STN) in advanced Parkinson’s disease. Met hods: We performed a double blind crossover evaluation of the efficacy of DBS of the STN in the “off” service condition in 10 patients with Parkinson’s disease a. Assessments the Unified Parkinson’s Disease Scale (UPDRS) part III (motor) and two timed tests (arm tapping and walking). of the effect of stimulation in the off and on drug states preoperatively and at 1 and 4 years postoperatively was also conducted. The latter assessment include d the UPDRS parts II (activities of daily living) and III (dyskinesia scale and global assessment) as judged by the patient and examiner. The mean amount of lev odopa daily dose at base line, 1 year, and 4 years after surgery was compared. R esults: A significant (p <0.04) effect of stimulation was observed in the overa ll group regarding both the UP DRS motor and the timed tests. Open evaluation als showed a significant benefit of STN DBS with respect to preoperative assessmen t in both the motor and activities of daily living scales, dyskinesia scale, and in global assessment. Levodopa daily dose was reduced by 48 % and 50% at 1 and 4 years, respectively. There was no difference between the 1 and 4 years evaluati ons in any of the parameters evaluated. Complications due to stimulation were mi nor. Conclusions: DBS of the STN provides a significant and persistent anti par kinsonian effect in advanced Parkinson’s disease 4 years after surgery.