The accuracy of image to patient registration is a critical issue in the intraoperative use of frameless stereotaxic instruments for surgical guidance. This study was performed to assess the accuracy of image to head phantom registration using several standard registration techniques and a clinical frameless stereotaxic instrument. Two types of radioopaque fiducial markers were fixed to a plastic head phantom, and a computed tomography scan of the phantom was performed in the routine fashion. Image to phantom registration was carried out using fiducial markers, fiducial markers plus surface fit, anatomic landmarks, and anatomic landmarks plus surface fit. After each registration, linear inaccuracy measurements were performed for each of 32 markers. Each registration was performed 10 times, and the overall mean error measurements and anterior and posterior error were computed and compared. The overall mean error was smallest for the fiducial registration alone (2.07 mm). The magnitude of error increased significantly for posterior locations for all other registration techniques, but it was not significantly increased for the fiducial registration method. For this hardware configuration, registration with surface-applied fiducial markers is measurably more accurate than registration with surface anatomic landmarks. The addition of surface points to the fiducial registration does not increase the registration accuracy but, in fact, increases the degree of error.