A prolonged QT interval is an important prognostic indicator for cardiac arrhythmias and sudden death. The conventional QT interval measurement, however, includes in its measure the cardiac depolarization (QRS) as well as the cardiac repolarization (JT) intervals. To evaluate the relative contribution of the depolarization and the repolarization time prolongation to the prolonged QT interval in patients with intraventricular conduction delay (IVCD), the QRS, QT, and JT intervals were measured in 72 subjects with various types of IVCD. The observed intervals in IVCD subjects were compared to similar intervals in 33 healthy individuals in whom there was no evidence for intraventricular conduction abnormalities. The QTc (QT interval corrected for heart rate) in subjects with IVCD were 445 +/- 6.8 msec (mean +/- SEM) in those with LAD, 470 +/- 9.1 msec with RBBB, and 489 +/- 6.9 msec with LBBB. All of these intervals were significantly prolonged compared to 430 +/- 4.3 msec in the control group. The prolongation of QTc interval in each category of IVCD subjects was entirely secondary to a prolonged depolarization time, as the repolarization intervals were not significantly different from those observed in the control group (F = 0.5, p = NS). These observations may provide an explanation for the differential prognosis for subjects with prolonged QT interval with prolonged repolarization time as compared to those with prolonged QT interval with prolonged depolarization time.