A hundred tracings of ventricular tachycardia (VT) belonging to 85 patients with myocardial infarction (MI) were compared with 70 cases of incessant, benign, idiopathic VT. The two groups of tracings differed in terms of QRS axis, most often normal in idiopathic VT (75%) and outside normality in MIVT (74%). The sum of QRS amplitude in unipolar limb leads was greater in idiopathic VT (4.3 +/- 1.3 mv, mean +/- S.D.) than in MIVT (2.6 +/- 0.8 mv, P less than 0.001). The QRS width was also different: 135 +/- 11 ms in idiopathic VT vs. 171 +/- 32 ms in MIVT (P less than 0.001). The QRS morphology in MIVT was characterized by the presence of a QR pattern in leads other than VR, or a QS pattern in V5-V6. These two aspects were constantly absent in idiopathic VT, and they were present in 89% of MIVT. In only 38 MIVT tracings were the ECG signs of MI observed in the same leads during sinus rhythm and during VT. In 51 MIVT tracings the location of the MI indicated by the VT tracing differed from that displayed in sinus rhythm. Rather than indicating an extension of the infarcted area not apparent in the tracings in sinus rhythm, such a discrepancy suggests that the QRS pattern during VT strongly depends on the point of origin of the VT. Conversely, this explains why the morphology of the QRS is an unreliable means for localizing the VT origin if the location of the MI is not taken into account. We conclude that both factors should be taken into consideration, and this might theoretically permit a better though complex approach to the VT origin in coronary heart disease using surface tracings.