The aim of this study was to systematically review the data published on deep brain stimulation (DBS) of the globus pallidus internus (GPi) in Parkinson's disease (PD), and to determine its efficacy and optimal stimulation parameters. Only 22 of 4,648 articles fulfilled the inclusion and exclusion criteria of the study. The data were analysed using the Wilcoxon test. For 327 patients who underwent GPi-DBS, the preoperative baseline Unified Parkinson's Disease Rating Scale (UPDRS) score, off-medication, mean was 52.7 (range 26.5-77.2). The postoperative UPDRS score, off-medication/on-stimulation, mean was 33.7 (range 18.7-46.2). The delta mean (the difference in mean UPDRS score between baseline and maximum follow-up) was 19.1 (range -2.2 to 36.5) (p<0.001). When the electrical parameters were compared against the delta UPDRS score, the analysis showed that only frequency was correlated with motor improvement (R(2)=0.42, p<0.05). Thus, GPi-DBS is a highly effective target for neuromodulation in PD. However, we found that significant clinical improvement (>50% delta UPDRS score) in PD is achieved at an amplitude of between 2.0V and 3.5V, a pulse-width between 75 micros and 300 micros and a frequency between 100Hz and 190Hz.