A three-dimensional computer graphic program has been developed to guide the surgeon through the resection of a brain tumor. It consists of a video camera mounted on a stereotactic frame which visualizes the operative field in real time. Superimposed on the video monitor is a computer generated model of the tumor or other resection target mass. The mass can be displayed in several modes. The computer image of the mass is adjusted to correspond to the surgeon's eye view, with appropriate settings for the stereotactic apparatus indicated. Prior to incision, the image of the mass is superimposed on the scalp to guide placement and size of the scalp and bone flaps. As the resection proceeds, a cross-section of the mass is illustrated on the video picture of the operative field. The distance from the camera to the surface is measured, and the cross-section at that depth is displayed. A small window indicates the surgeon's eye view and the tangential view of the mass with an indication of the cross-section currently being displayed. As resection proceeds, the distance to the bottom of the resection cavity is measured repeatedly. The image is updated to indicate only that level of the tumor under resection. As the far side of the tumor is encountered, the mass disappears from the monitor, signaling completion of the resection.