We performed percutaneous microwave co-agulation therapy (PMCT) using a new microwave electrode and microwave generator to lessen the number of microwave electrode insertions for hepatocellular carcinoma (HCC) measuring <or=3.0 cm in diameter. In this study, we assessed the efficacy of this new system. Fifty-two patients with liver cirrhosis and HCC measuring <or=3.0 cm in diameter (tumor size: <or=2 cm, 27 patients; >2 to <or=2.5 cm, 13 patients; >2.5 to <or=3 cm, 12 patients) underwent PMCT using a new microwave coagulation system under ultrasonographic guidance. Forty-two of 52 patients showed complete necrosis of the tumor lesion with a treated margin >or=5 mm on dynamic computed tomography. Necrosis of HCC and the non-cancerous area surrounding the tumor was obtained by a single needle insertion in 23 patients (tumor size: <or=2 cm, 23 patients), by two needle insertions in 11 patients (tumor size: <or=2 cm, 4 patients; >2 to <or=2.5 cm, 5 patients; >2.5 to <or=3 cm, 2 patients), and by three needle insertions in 8 patients (tumor size: >2 to <or=2.5 cm, 5 patients; >2.5 to <or=3 cm, 3 patients). During the follow-up period of 5-34 months, all patients remained alive. Among the patients showing complete necrosis of HCC with a treated margin >or=5 mm, we have not detected local recurrences. On the other hand, 4 of the 10 patients who could not obtain the treated margin of >or=5 mm, experienced a local recurrence. No fatal complications were observed. However, bile duct stricture was observed in 2 patients. The new microwave coagulation system can induce extensive necrosis with a small number of microwave electrode insertions. This system may enhance the efficacy of PMCT for HCC measuring <3.0 cm in diameter.