Active tracking or profiling coils mounted on catheters provide high signal. Other devices in the vicinity can alter the signal acquired by the catheters. The effects of guidewires on the signal from actively tracked catheters were investigated in phantom and in vivo. A standard metallic guidewire and a MR safe guidewire with a passive marker were introduced through the catheter and tested for different positions at the tip. The signal loss was substantial for both guidewires. When the metallic guidewire passed through the catheter tip, the signal was unstable, whereas the MR safe guidewire did not cause any distortions.