Accuracy of a stereotactic operating microscope, by which imaging data may be superimposed on the operative field without a stereotactic frame, has been most limited by the resolution of imaging information. Using newer algorithms and pilot pole calibration of the digitizer, an error in registration of 2 mm and in contour display of 3 mm has been demonstrated. Greatest utility of the system clinically has been in providing navigational guidance to small lesions undergoing resection.