Percutaneous microwave coagulation therapy (PMCT) has recently been introduced as a new treatment for hepatocellular carcinoma (HCC) in Japan. This study was performed to evaluate its efficacy and safety. Thirteen patients with 17 nodules of unresectable HCC were subjected to PMCT under ultrasonic guidance. The tumors ranged from 1.2-4.4 cm in size. Assessment of the efficacy of PMCT was made by follow-up with dynamic computed tomography (CT). In the patients with small HCC (< or = 2.0 cm), 8 of 10 nodules (80%) showed complete remission after PMCT. In small nodules located on the liver surface, 3 out of 4 nodules (75%) showed complete remission. However, in the patients with larger HCC (> or = 2.1 cm), 5 out of 7 nodules developed local recurrence after PMCT. Regarding assessment of the necrotic area after PMCT, dynamic CT revealed enhancement that was possibly caused by congestion of the liver parenchyma surrounding the area of necrosis due to PMCT in the early phase of the treatment. Therefore, the necrotic area must be assessed carefully. Although a slight heat sensation and/or pain during microwave irradiation (a common effect of PMCT) occurred in all patients, there were no serious adverse effects. Complete remission of small HCC (< or = 2 cm in diameter) can be achieved with PMCT alone, but there seem to be limitations to its effectiveness with larger HCC (> or = 2.1 cm). There were no serious adverse effects from PMCT and the therapy can be safely carried out even in patients with poor liver function.