Catheter electrical ablation of ventricular tachycardia (VT) was attempted in 33 patients who had recurrent unimorphic VT refractory to 3.7 1.2 (mean + SD) antiarrhythmic drugs. Their mean age was 56 + 14 years. Twenty-two patients had coronary artery disease, six had other types of heart disease, and five had no structural heart disease. The mean left ventricular ejection fraction was 0.34 + 0.17. Thirty patients had only one documented morphologic type of spontaneous VT, whereas three patients had more than one. One to four shocks of 100 to 300 J each were delivered to the endocardial exit site of VT, as identified by endocardial activation mapping and pace-mapping. In each patient endocardial activation at the exit site of VT preceded the onset of the QRS complex (mean activation time -50 30 msec). Pace-mapping was possible in 26 patients, and in all but two patients the QRS complexes during VT and during pacing at the exit site of VT were very similar in at least 10 of 12 electrocardiographic leads. In 29 patients, shocks were delivered between an endocardial electrode (cathode) and a patch electrode on the chest wall (anode). Seven patients (including three who first received shocks using an external anode) whose VT originated in the septum received transseptal shocks between two electrodes positioned on either side of the septum. The procedure was successful in 15 patients (45%), who had no recurrence of VT either on no antiarrhythmic therapy or on the same regimen that was ineffective before ablation, over a follow-up period of 15.5 + 10 months (range 5 to 35). The ablation attempt was unsuccessful in 18 patients (55%). There were no significant differences in clinical and electrophysiologic variables between patients with and without a successful outcome. Seven nonfatal complications occurred in six patients: sustained nonclinical VT immediately after the shock, ventricular fibrillation on days S and 6 after ablation, neurologic deficits (n = 2), atrioventricular block (n = 2), and brachial artery thrombosis. In conclusion, catheter electrical ablation of VT