In orbital surgery, deep lesions must be operated on through extremely small approaches. We developed a method (Computer Assisted Surgery or CAS) designed to achieve safer and more precise surgery by providing highly accurate intraoperative information regarding the location of the surgical instrument. The device involved consists of a high-capacity computer that processes computed tomographic (CT) data in real time, connected to a mechanical measuring arm attached to the surgical instrument. From the CT data, the computer calculates a three-dimensional reconstruction and displays the tip of the instrument as a reticle in this model. Using the CAS system, the surgeon is continuously apprised of the precise position of the surgical instrument vis-a-vis all relevant anatomical structures. We have used CAS successfully in 21 cases of orbital surgery. We have found the system especially useful in dealing with deep-seated tumors, either for biopsy or surgery; in performing orbital and optic nerve decompression; in localizing deep-seated foreign bodies; and in all cases of impaired orientation due to severe bleeding, tumor-destroyed landmarks, or preoperated anatomy. Although the number of patients is too small to yield statistically significant data allowing a comparison of patient outcomes with and without using the system, we have just begun a large prospective clinical trial designed to provide such data.