Background: Several ECG criteria have been proposed to differentiate left from right ventricular outflow tract (OT) premature ventricular contractions (PVC)/ventricular tachycardia (VT); however, differentiating PVC/VT in left bundle branch block (LBBB) remains challenging. Individual patient differences in cardiac rotation, lead positions, and chest wall size may limit the accuracy of the ECG algorithm. We hypothesized that correcting for sinus rhythm (SR) precordial transition would aid in OTVT localization. Methods: We analyzed the surface ECG patterns of 16 patients (8 men, 8 women; age, 55.7 ± 22.2 years) with right ventricular (RV) OTVT and 8 patients (7 men, 8 women; age, 50.1 ± 16.6 years) with left ventricular (LV) OTVT who underwent catheter ablation. SR and VT morphologies were measured with the same 12-lead ECG system. V2 transition ratios were determined by calculating the percentage R wave during VT (R/R+S) VT divided by the percentage R wave in SR (R/R+S) SR. We also determined the V1 and V2 R/S ratios, the time from V2 R-wave onset to its peak (V2Rp), V1- and V2 R-wave durations, S wave in V5 and V6, the precordial transitional zone, and the lead I ECG QRS pattern during PVCs/VT. Results: The V2 transition ratio was significantly greater for LVOT vs. RVOT (1.50 ± 0.69 vs. 0.18 ± 0.11; P = 0.0004). A V2 transition ratio >1.1 predicted an LVOT origin with a sensitivity of 83% and specificity of 88%. Time to V2Rp was significantly greater for LVOT vs. RVOT (82.50 ± 6.12 vs. 41.2 ± 9.90 ms; P = 0.0003). A V2Rp time >70 ms predicted an LVOT origin with a sensitivity of 100% and specificity of 100%. Conclusions: Electrocardiographic measures, the V2 transition ratio, and V2Rp time can reliably distinguish between an RVOT and LVOT origin in patients with LBBB. These measures may be useful in planning the ablation strategy.