A three-dimensional magnetic field digitizer has been interfaced the COMPASS Stereotactic System to act as a measuring device aiding in computer-assisted volumetric procedures. Reference points on the stereotactic headholder are used to create a transformation matrix to convert the digitizer coordinates to stereotactic coordinates, allowing the location of the stylus to be displayed on CT and reconstructed tumor volume images to maintain the surgeon's orientation. This technology is an adjunct to and employs treatment planning software of the system to calculate a target and determine a safe trajectory to a lesion. Environmental effects of the magnetic field have been studied to determine overall accuracy and reliability of the system. A device was developed to map the magnetic field in efforts to compensate for environmental effects. The device has been utilized on five procedures. We have found it to be a useful tool in aiding the surgeon in locating the surgical field preoperatively and maintaining orientation intraoperatively.