BACKGROUND: We aimed to investigate the possible factors determining reduction of dopaminergic drugs in patients who received bilateral subthalamic nucleus (STN) deep brain stimulation (DBS). METHODS: We studied 33 consecutive Parkinson's disease (PD) patients who underwent bilateral STN DBS between March 2002 and April 2006. Patients were assessed at baseline and 6 months and 1 year after surgery. RESULTS: The dose of dopaminergic drugs was significantly decreased by 16.5% (p<0.05) and 14.9% (p<0.05), at post op 6 months and 12 months respectively. The reduction rate of dopaminergic drugs after bilateral STN DBS was positively correlated with the dopaminergic drug dosage at baseline (p<0.05) and was negatively correlated with the scores of "off"-period UPDRS II (p<0.05), III (p<0.001), and total UPDRS (p<0.001) at baseline. The difference of UPDRS scores between "on" and "off" periods on the levodopa challenge test was negatively correlated with the reduction rate of dopaminergic drug dosage after bilateral STN DBS (p<0.001). CONCLUSIONS: It is likely that the main determining factors for the reduction of dopaminergic drug dosages after bilateral STN DBS in advanced PD are the UPDRS III score during "off" periods, difference of the UPDRS III score between "on" and "off" periods on the levodopa challenge test and dose of dopaminergic drugs at baseline.