Previous studies in patients with sustained ventricular tachycardia (VT) have demonstrated the efficacy of surgical and catheter-mediated ablative procedures based on activation mapping during VT. Since extensive preoperative or intraoperative mapping may be impractical due to time constraints or patient intolerance, we sought to define characteristics of the 12-lead electrocardiogram (ECG) during VT that could suggest a particular endocardial region of origin and thus facilitate mapping studies. Endocardial mapping was performed during 182 VTs in 108 patients with prior myocardial infarction of either the anterior or inferior wall. Endocardial sites of origin (sites from which greater than or equal to 40 msec of presystolic electrical activity was consistently recorded) were identified with use of catheter (154 VTs) or intraoperative (85 VTs) activation mapping (both methods used in 57 Vts). Twelve-lead ECGs obtained during these VTs were characterized by four features: location of infarction, bundle branch block type configuration, quadrant of QRS axis, and precordial R wave progression pattern. A specific combination of these four features was associated with a particular endocardial region containing the mapped site of origin in 87 VTs (48% of total). An association (greater than or equal to 70% positive predictive accuracy) was more likely to be found in the presence of left, as opposed to right, bundle branch block type patterns (53/73 [73%] vs 34/109 [31%]; p less than .001) and in the presence of VT related to inferior, as opposed to anterior, infarction (40/54 [74%] vs 47/128 [37%]; p less than .001).(ABSTRACT TRUNCATED AT 250 WORDS)