BACKGROUND Previous studies have shown that the size of lesions produced by radiofrequency catheter ablation correlates with the temperature and surface area of the electrode-tissue interface. The purpose of the present study was to compare the effects of ablation using very large distal electrodes (8F, 8 and 12 mm long) with those made by a conventional radiofrequency ablation catheter (distal electrode 8F, 4 mm long). RESULTS Each catheter had a thermistor in the tip of the distal electrode. Radiofrequency energy (500 kHz) was supplied by a generator that continuously monitored temperature and produced up to 100 W. In 10 dogs, each of the three ablation catheters were introduced percutaneously and positioned under fluoroscopic guidance at disparate left ventricular endocardial sites. Radiofrequency power output was titrated to achieve a temperature of 80 degrees C for 60 seconds at each ablation site. The power required to produce a steady-state temperature of 80 degrees C was directly proportional to electrode size (15 +/- 7, 46 +/- 15, and 62 +/- 32 W using the 4-mm-, 8-mm-, and 12-mm-long electrodes, respectively). Lesions produced by the 8-mm electrode were nearly twice as deep (11 +/- 2.4 versus 6 +/- 1.2 mm, P < .001) and four times as large (905 +/- 410 versus 210 +/- 100 mm3, P < .001) as those made with a conventional 4-mm electrode. Lesions produced by the 12-mm electrode were intermediate in size (depth, 8 +/- 1.2 mm; volume, 465 +/- 225 mm3) and sometimes were associated with charring and crater formation. Ablation with the larger electrodes caused a drop in arterial pressure and more ventricular ectopy than ablation using a 4-mm distal electrode. CONCLUSIONS Thermistor-equipped elongated ablation electrodes coupled to high-power outputs can reproducibly produce lesions approximately 1 cm in diameter. This system may prove useful for ablation of ventricular tachycardias in patients with coronary artery disease.