This paper describes the use of three-dimensional computer graphics to display structures visible in computerized tomography (CT) scans and to accurately determine optimum stereotaxic probe placement relative to those structures. A prototype Lucite stereotaxic frame designed for use in CT body scanners was fitted with a phantom consisting of small Lucite spheres representing intracranial tumors with diameters of 6 to 19 mm. A series of CT scans was obtained of the frame and phantom together. Edge outlines of the spheres were extracted from each scan in the series. A three-dimensional visual representation of the spheres was obtained by displaying their outlines from all CT scans. A superimposed visual simulation of the stereotaxic frame was adjusted interactively using several analog dials in order to simulate trajectory choice and probe depth before actual surgery. The frame settings and probe depth calculated by the graphical computer were then applied to the actual prototype frame in order to assess the accuracy of this combined CT-computer graphics approach to stereotaxic localization. The results of 22 such experiments and the implications for future clinical use of this new and precise localization technique are discussed.