The present study analyzes the electromagnetic interference produced on visual display units (VDUs) in domestic and industrial environments. The main sources of disturbance may be identified in three-phase lines, unbalanced currents, currents in earthing systems, proximity of power installations, proximity of railway tracks, and presence of harmonics on the neutral conductor. Magnetic-field interference for PCs is practically limited to the deflection of the cathode-ray tube (CRT) electron beam. The vertical magnetic field determines a horizontal electron beam motion. The distortion depends linearly on magnetic field intensity and on the difference between the frequency of the CRT vertical raster scan and the frequency of the interfering magnetic field. Herein are analyzed two actual cases of interference produced by the DC-supply line of a metropolitan tram network and by the public electric power distribution network.