The majority of patients with primary or metastatic liver tumors are not candidates for resection because of the size, location, or multifocality of their tumors, or because of inadequate hepatic function related to cirrhosis. Radiofrequency ablation (RFA) is an evolving technique for treating patients with unresectable primary or metastatic liver cancers. After obtaining the approval of our institutional review board for this study, 12 patients with HCC and 6 patients with metastatic liver tumors were treated using the LeVeen RF ablation system at the Department of Surgery of Osaka National Hospital between March 2000 and February 2002. Informed consent was obtained from all patients. Ultrasound-guided RFA was done during open surgery. In 12 patients, RFA was performed during laparotomy, while in 6 patients it was done transdiaphragmatically during thoracotomy. All treated tumors showed complete necrosis on imaging after the completion of RFA. After a median follow-up period of 288 days, the tumor had recurred in 5 out of 18 patients, and the median overall survival rate was 362 days. No deaths or major complications occurred in these 18 patients. Liver function tests (ALT, AST, GGT) that were elevated after RFA returned to baseline in most patients by day 7. In 5 patients who underwent RFA at laparotomy, bile leakage and liver abscess developed. There were no cases of bile duct injury or liver abscess in the patients receiving transdiaphragmatic RFA. In conclusion, transdiaphragmatic RFA during thoracotomy is a safe, well-tolerated, effective treatment for unresectable hepatic malignancies.