The VISLAN system is based on one of the newest developments in optical localiser tracking, where the localiser reflects light shone on it (passive illumination) rather than emitting light. In experiments with a CT-compatible skull phantom we evaluated the VISLAN system in comparison with a widely used stereotactic system, the Cosman-Roberts-Wells frame (Radionics lnc). The mean Euclidean difference in localising points initially defined in CT images was 2.5mm across a wide range of target positions. We estimated the accuracy of the VISLAN system with the same phantom set up but independently of the stereotactic frame, and showed a mean Euclidean error of 1.0mm. We concluded that the majority of the discrepancy between the two systems was accounted for by the inaccuracy of the CRW frame. It would be inappropriate to regard the frame based systems as a satisfactory gold standard for estimating the accuracy of VISLAN. However, frame-based and frameless guidance may be needed in the same operation, and it is important for such systems to be compared.