As computer-interactive technologies become more widely used in neurosurgery, radiology, and radiation therapy, the need for an optimum skull fiducial marker system increases. In the past, intracranial localization methods required precisely machined metal frames and rigid pin fixation to the skull. Recently, this function has been performed using "frameless" computer-based systems that calculate brain position relative to a series of external reference points, the most accurate of which are screwed directly into the skull. A penetrating fiducial marker system, however, is not well suited for applications requiring multiple volume registrations over an extended time period. We describe a new skull fiducial marker system that attaches to the maxillary teeth and can be used repeatedly on different occasions. A curved bar, known as a Banana Bar (BB) extends backward from a custom mouthpiece around the side of the patient's head; the bar contains sites of attachment for screw-in radiographic fiducial markers. Repositioning accuracy was quantitated using a photographic technique. A BB prototype was constructed and tested in three subjects. The BB weighs less than 100 g and can be comfortably held in position for up to 30 minutes. It takes less than 1 minute to screw in the mouthpiece and only seconds to secure the BB to the teeth. One hundred twenty photographic measurements were analyzed from 60 repositionings over a minimum 3-week period. Standard deviations for the measurement series ranged from 0.29 to 0.86 mm. Results suggest that the BB may be an inexpensive, efficient, and accurate method for providing the external reference points needed for a wide range of emerging computer-interactive applications.