Magnetic Field Interference.Introduction:High‐resolution electrocardiography, with or without signal averaging, requires extremely low levels of extraneous noise pickup. A significant amount of such interference is transmitted to the patient electrode wires as a magnetic (as opposed to electric) field, which is unresponsive to many commonly used shielding methods.Methods and Results:To investigate the source and quantify the strengths of magnetic fields, we used a battery‐powered device of our own design and performed magnetic field measurements in various locations in two hospitals. We found that significant sources of magnetic interference were cathode ray tube defiection coils, electric motors, and power transformers on wall‐mounted ophthalmoscopes or on the electrocardiographic equipment itself. The lowest noise level could be produced by running the patient electrode wires twisted closely together and by distancing the magnetic field sources, including electrocardiographic equipment.Conclusion:We conclude that magnetic fields at typical locations in modern hospitals are usually of such strength as to cause significant interference in high‐resolution electrocardiography. To reduce such an interference, the patient electrode wires should be twisted together and all suspected magnetic field sources, including the signal‐averaging equipment itself, should be placed as remotely from the patient as possible. (J Cardiovasc Electrophysiol. Vol. 3. pp. 34–39, Fe