The purpose of this study was to demonstrate the efficacy of bipolar radiofrequency ablation (RFA) using cooled-wet electrodes inducing coagulation in ex vivo bovine livers and in in vivo canine livers. In ex vivo experiments, 20 coagulations were created by monopolar (group A), and bipolar RFA (group B) using a 200 W generator (Valleylab) and one or two cooled-wet electrodes. In in vivo experiments, one coagulation was created by bipolar RFA in each of eight dogs via laparotomy. In ex vivo and in vivo experiments, RF was applied to one or two electrodes at 100 W for 10 min. The dimensions of the coagulations were compared in the two groups. In ex vivo experiments, the mean volumes of the coagulations produced in group B (54.0+/-16.5 cm3) were greater than those produced in group A (33.9+/-12.7 cm3) (P=0.007). In in vivo experiments, bipolar RFA produced a coagulation of 39.4+/-15.6 cm3 without a major complication. The present study showed that a RF electrode system using two cooled-wet electrodes in the bipolar mode created larger coagulation volumes than the monopolar mode, and this system can be used to create large coagulation without major complications.