The magnetic fields caused by the human heart's electrical activity were simultaneously recorded with a multichannel superconducting quantum interference device (SQUID) system (Krenikon) for 1-10 min in 45 patients. 31-37 magnetic channels were recorded simultaneously with the electrocardiogram (ECG) and respiration. Comparison of a magnetic index and the Sokolow-Lyon index with echocardiographic findings in the quantification of left ventricular hypertrophy demonstrated the superiority of the magnetocardiogram (MCG) as compared with the ECG. The magnetocardiographic investigation of patients with Wolff-Parkinson-White (WPW) syndrome, ventricular extrasystoles, ventricular tachycardia, and paced ventricular arrhythmias demonstrated that multichannel magnetocardiography permits the non-invasive three-dimensional localization of arrhythmogenic tissue with high spatial accuracy.