With the invention of miniaturized electromagnetic digitizers comes a variety of potential clinical applications for computer aided interventions using flexible instruments; it has become possible to track endoscopes or catheters within the body. To evaluate the reliability of a new commercial tracking system, we measured the systematic distortions induced by various materials such as closed metallic loops, wire guides, catheters and ultrasound scan heads. The system under evaluation was the electromagnetic tracking system Aurora (Mednetix/CH, NDI/Can); data were acquired using the serial port of a PC running SuSE Linux 7.1 (SuSE, Gmbh, Nuernberg). The objects suspected to cause distortions were brought into the digitizer volume. Beside this, we evaluated the influence of a C-arm fluoroscopy unit. To quantify the reliability of the system, the miniaturized sensor was mounted on a nonmetallic measurement rack while the transmitter was fixed at three different distances within the digitizer range. The tracker is more sensitive to distortions caused by materials close to the emitter (average value 13.6 mm +/- 16.6mm) for wire loops positioned at a distance between 100 mm and 200 mm from the emitter). Distortions caused by materials near the sensor (distances smaller than 100 mm) are small (typical error: 2.2 mm +/- 1.9 mm) in comparison to the errors of other electromagnetic systems published in an earlier study of our group where we found an average error of 3.4 mm. Considerable distortions are caused by the C-arm fluoroscopy unit and limits the reliability of the tracker (error: 18.6 mm +/- 24.9 mm). The US scan head was found to cause significant distortions only at a distance between the emitter and the scan head less than 100 mm from the emitter in contrast to the average error of 3.8 mm +/- 6.3 mm at distances greater than 100 mm. Taking into account that significant distortions only occur in the presence of metallic objects close to the emitter, these results indicate the opportunities which are now available in surgical applications where flexible instruments may need to be monitored within the patient.