The operation of six degree-of-freedom electromagnetic trackers is based on the spatial properties of the electromagnetic fields generated by three small coils. Anything in the environment that causes these fields to be distorted will result in measurement noise and/or errors. An experimental investigation was undertaken to measure the effect of external fields present in a typical working environment (namely mains and computer monitor fields) and the presence of metals (25-mm cubes of various types of metals, a large steel bar, and a large steel sheet). A theoretical model is proposed to explain the observations. Two devices were used in this investigation: a Polhemus Fastrak and an Ascension Flock of Birds.