To compare the modified precordial leads MCL1 and MCL6 with the conventional precordial leads V1 and V6 and assess the diagnostic accuracy of selected leads for continuous bedside electrocardiographic (ECG) monitoring, 121 wide QRS complex tachycardias were recorded from 92 patients during cardiac electrophysiologic study. As ascertained from intracardiac recordings, 86 tachycardias were ventricular and 35 were supraventricular with aberrant conduction. Early or late peaking of the predominant QRS deflection in lead MCL6 or V6 proved valuable in diagnosing wide complex tachycardia. An interval of less than or equal to 50 ms from the onset of the QRS complex to the predominant peak (or nadir) indicated supraventricular tachycardia; an interval of greater than or equal to 70 ms indicated ventricular tachycardia. The QRS complexes in leads MCL1 and MCL6 were comparable to those in leads V1 and V6 during sinus rhythm. Significant discrepancies in QRS configuration occurred between the modified and conventional precordial leads during ventricular tachycardia, especially between leads MCL1 and V1; however. these differences did not affect diagnostic accuracy. A single MCL1, V1, MCL6 or V6 lead was equally valuable in the diagnosis of wide complex tachycardia and far superior to a single lead II. A combination of leads (MCL1 + MCL6), (V1 + V6), (V1 + I + aVF) or (V1 + V6 + I + aVF) was superior to a single lead or the routinely monitored lead V1 + II combination.