Internal landmarks for the subthalamic nucleus (STN) have been used in past. This study uses a yet-unused internal landmark to refine STN targeting.To determine the effect of the width of the interpeduncular cistern (IPC) on STN targeting during placement of deep brain stimulation (DBS) for Parkinson disease.Fifty consecutive patients with Parkinson disease underwent 90 STN DBS implantations. X, Y, and Z coordinates for the tip of the DBS electrodes and the active contact were recorded. Internal landmarks such as width of the third ventricle, width of IPC at a predefined point, and anterior commissure-posterior commissure length were measured. Statistical analysis was done using linear regression analysis and Pearson correlation coefficient.The average IPC diameter at the predefined point was 7.59 mm (range, 5- to 14 mm). Average X, Y, and Z coordinates for the location of the tip of lead were 11.5, -3.5, -5.4, and those of the location of active contact were 12.5, -1.9, -1.4 from the midcommissural point. The mediolateral location of the tip of the DBS as well as the location of the active contact for long-term stimulation were greatly dependent on IPC width (r = 0.83) (P = .0022).The width of the IPC is a strong predictor of laterality of STN DBS electrode placement in patients with Parkinson disease. It can be used as an additional internal landmark for refining STN targeting using the simple formula X coordinate for STN target = 0.6 × IPC width + 7 mm.