Background— The proximity of the outflow tracts (OTs) frequently results in an overlap in surface electrocardiographic features of ventricular arrhythmias originating from this anatomic region, particularly when the transition occurs in lead V ₃ . In addition, no reliable criteria to discriminate between a right ventricular OT (RVOT) and a left ventricular OT (LVOT) site of origin (SOO) are derived from intracardiac mapping. Methods and Results— A series of 15 patients underwent ablation because of OT ventricular arrhythmias having a V ₃ transition, and a septal earliest activation on the RVOT was included in the study. Electrocardiographic and mapping data were collected to analyze accuracy in predicting the RVOT versus the LVOT SOO of the ventricular arrhythmia. A 10-ms isochronal map area in the RVOT was smaller in the RVOT SOO group (1.2 [0.4–2.1] versus 3.4 [2.4–3.9] cm ² , respectively; P =0.004) and had a shorter perpendicular diameter (13 [7–17] versus 28 [20–29] mm; P =0.001) and a higher longitudinal/perpendicular axis ratio (1.04 [0.95–1.11] versus 0.49 [0.44–0.57]; P =0.001). A 10-ms isochronal map area >2.3 cm ² predicted an LVOT origin with 85.7% sensitivity and 87.5% specificity, whereas a longitudinal/perpendicular axis ratio <0.8 predicted an LVOT origin with 100% sensitivity and 100% specificity. Electrocardiography-derived parameters showed lower values of sensitivity and specificity. The distal coronary sinus activation mapping did not permit distinction between RVOT and LVOT SOO. Conclusions— The 10-ms isochronal map area and the longitudinal/perpendicular axis ratio accurately predict the RVOT versus the LVOT SOO in patients with OT ventricular arrhythmias, a V ₃ transition, and a septal earliest activation.