Computer-assisted surgery (CAS), a new navigation aid for skull-base surgery, is discussed. The system described combines 3-D coordinate measurement techniques, voxel processing methods, and pseudo-3-D image presentations to support preoperative planning of therapy, path-finding during the operation itself, and postoperative therapy control. The surgeon employs a hand-guided electromechanical 3-D-coordinate digitizer to locate points of interest within the operative field. The coordinates measured this way are correlated with a voxel model of the object gained by a preceding computed-tomography examination. With a prototype system the accuracy of this method has proven to be better than +or-1 mm. The system has been successfully applied in more than 60 ear-nose-throat operations and four neurosurgical procedures. The computer hardware and integration of the system into an experimental picture archiving and communication system are discussed. >