Using a microwave tissue coagulator, we attempted intraoperative, ultrasound-guided cauterization and thermocoagulation of metastatic liver tumors. The procedure, safety and efficacy of this therapy were analyzed. During the past 2 years, this therapy was used in 10 patients with unresectable metastatic liver tumors. During surgery, monopolar electrodes were inserted with an ultrasonic guide. One round of cauterization lasted for 10 seconds at 100 W. It was repeated until the tissue around the tumor became hyperechoic. This therapy was used for 67 tumor lesions in total (2-22 lesions/patient). No complications were observed during or after this therapy, except for a case complicated by liver abscess. The anti-tumor effect of the therapy was assessed by histological examination of resected liver tissues. Tumor coagulation and necrosis were observed consistent with the cauterized area. The response to this therapy, assessed by diagnostic imaging, was CR (complete remission) in 3 cases, PR (partial remission) in 3, NC (no change) in 3 and PD (progressive disease) in 1. Thus, the efficacy (the percentage of cases showing CR or PR) was 60%. Since this therapeutic approach acts directly on tumors and is simple and safe, it seems to provide a useful means for intraoperative regional treatment of tumors.