Magnetocardiography (MCG) is a new modality which utilizes superconducting quantum interference devices for the detection of the weak magnetic fields (picoTesla range) generated by the heart's electrical currents. The magnetic field map picture, which is created by the measurements of the magnetic field, reflects the electrophysiological state of the heart. When there is an abnormality in cardiac depolarization or repolarization, such as in ischemia or myocarditis, this is reflected in an abnormality in the magnetic field map (7). Although MCG and ECG both measure the cardiac depolarization and repolarization patterns, they have fundamental differences. MCG is most sensitive to tangential currents whereas ECG is most sensitive to radial currents in relation to the chest surface (7-10). Cardiac abnormalities, which interfere with the normal activation and deactivation sequence, such as may occur in acute rejection, increase the contribution of tangential currents. In addition, the MCG detects the vortex currents which are not evident by ECG. Finally, the MCG is less affected by conductivity variations caused by lungs, skin, and muscles and there is no skin electrode contact problem since the device does not come in direct contact with the skin. Therefore, the MCG may be able to detect differences in depolarization and repolarization in a different manner and with a higher sensitivity than the ECG. We speculate that the variation in the magnetic field pattern is due to early subtle changes in cellular mechanisms or metabolism. These changes create a heterogeneous repolarization pattern probably caused by local currents appearing at the border zones between normal and diseased myocardium. This implies that changes in the MCG may appear even earlier in the cascade of rejection than wall motion abnormalities, ECG changes, or changes in serum markers such as troponin.