Fifty-five patients with sustained ventricular tachycardia due to prior myocardial infarction underwent intraoperative endocardial activation mapping during ventricular tachycardia to guide subendocardial resection. The mapping data were analyzed to determine the pattern of endocardial activation during tachycardia. Of a total of 122 tachycardias, 101 had a pattern of activation assigned: in 90 (90%), endocardial activation spread centrifugally from a tachycardia site of origin, and 11 (10%) had a continuous loop of electrical activity around an aneurysm. All patients had at least one tachycardia having the centrifugal spread pattern. Tachycardias with a continuous loop pattern had a shorter mean cycle length than those with a centrifugal spread pattern (260 +/- 33 versus 338 +/- 81 ms, p less than 0.002) and a longer duration of endocardial activation relative to the tachycardia cycle length (100 +/- 0 versus 58 +/- 19%, p less than 0.001). There was no difference in preoperative patient characteristics, operative survival or cure of tachycardia between patients having any tachycardias of the continuous loop pattern and those having only centrifugal spread tachycardias. Thus, the vast majority of ventricular tachycardias in this group of patients are characterized by a centrifugal spread of endocardial activation from a site of origin less than 6 cm2 in size. Mapping-guided ablative surgery may remove the entire tachycardia circuit in these patients and a critical portion of the circuit in the minority of patients with continuous loop tachycardias.