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The electrocardiogram (ECG) patterns during pacing from the great cardiac vein (GCV) and the middle cardiac vein (MCV) are not well known.We recorded 12-lead ECGs during GCV and MCV pacing in 26 patients undergoing implantation of a cardiac resynchronization device. The left ventricular (LV) lead was passed down the GCV (n = 19) or MCV (n = 7) prior to moving it to a lateral or posterolateral vein for permanent implantation.Pacing within the GCV resulted in a left bundle branch block (LBBB) morphology with no or minimal R-wave in V1 in 14 patients and a right bundle branch block (RBBB) pattern (R > S in lead V1) in four patients. In one patient, lead V1 during GCV pacing was isoelectric (R = S). A more distal pacing site in the GCV yielded a LBBB pattern in all the patients. All leads placed in the MCV resulted in a LBBB configuration. An ECG pattern with a RBBB pattern was invariably recorded during LV pacing in 125 consecutive outpatients with biventricular pacemakers and LV leads in the posterolatral and lateral coronary veins. Knowledge of the ECG patterns from various pacing sites in the coronary venous system may be helpful for troubleshooting all types of pacing systems, especially those where the coronary venous pacing site is unintentional.