Subdural grid electrodes are implanted routinely for the pre-surgical work up of epilepsy. While different approaches are available, many centers, including ours, visualize electrode locations by co-registering pre-operative 3-D MR images with post-implantation 3-D CT images. This method allows the determination of the electrode positions in relation to the individual patient's anatomy, but does not easily allow comparison across patients. The goal of this study was to develop and validate a method for transforming electrode positions derived from 3-D CT images into standardized space. We analyzed data from twelve patients with subdurally implanted electrodes. Volumetric CT and MRI images were co-registered and then normalized into common stereotactic space. Electrode locations were verified statistically by comparing distances between the anterior commissure and a representative sampling of 8 electrode sites per patient. Results confirm the accuracy of our co-registration method for comparing electrode locations across patients.