Catheter electrical ablation using a relatively low level of energy--40 to 100 joules--was attempted in 12 consecutive patients with drug-refractory sustained ventricular tachycardia (VT). They had 19 monomorphic VTs, and ischemic heart disease was found as the underlying heart disease in one, nonischemic heart disease was found in nine, and no structural heart disease was seen in two patients. Electrical discharge was delivered at the site of the earliest endocardial activation in 17 VTs, and at the slow conduction area in two VTs. Among 19 VTs in 12 patients, 12 VTs (63%) in seven patients (58%) were successfully ablated and became noninducible during electrophysiologic study. There were no major complications, but transient atrioventricular block occurred in one patient and transient friction rub occurred in another. Delivered electrical energy and the time interval between the local electrogram and the surface QRS did not correlate with the clinical outcome of the procedure. However, "excellent" pace-mapped QRS morphology was obtained from the site of earliest activation or from the slow conduction area in 9 of 12 VTs in the successful cases but in only one of seven VTs in the unsuccessful cases. Low-energy catheter electrical ablation seems to be a satisfactory therapeutic procedure compared with the conventional method that uses an energy level of 200 joules or higher.