To determine the optimal treatment margin of local ablation therapy for small hepatocellular carcinoma (HCC), we investigated characteristics of microscopic satellite HCC nodules (msn) using resected livers, and the incidence of local recurrences in patients who underwent percutaneous microwave coagulation therapy (PMCT) according to whether or not an adequate treatment margin was achieved. We reviewed 117 single small HCCs (tumor size < or =3 cm ) resected with a > or =1-cm surgical margin. Among the surgically resected tumors, none of the msn were detected by preoperative imaging. When an msn was observed in the resected specimen, the maximum distance from the edge of the tumor to the msn was measured. Among the tumors sized < or =2 cm (n=66), the number having msn and the distance (mm) from the main tumor according to the degree of tumor differentiation were as follows: well (n=16), 1 (6.2%) and 1.4 mm; moderate (n=46), 5 (19.2%) and 5.8+/-1.2 mm; and poor (n=4), 1 (25.0%) and 4.8 mm. Among the tumors sized 2 to 3 cm (n=51), the corresponding results were: well (n=11), 3 (27.2%) and 3.5+/-2.2 mm; moderate (n=36), 6 (16.7%) and 5.4+/-1.4 mm; and poor (n=4), 1 (25.0%) and 4.9 mm. Of the 112 vascular tumors, 17 (15%) had msn. The 5 avascular tumors had no msn. Among the patients who underwent PMCT with a treatment margin measuring <5 mm, local recurrence occurred in 2 of 25 patients whose original tumor was < or =2 cm, and 3 of 25 (12%) patients of those with tumors of 2 to 3 cm. No patient with a tumor < or =3 cm and a > or =5-mm treatment margin suffered a local recurrence. The results of this study suggest that the incidence of local recurrence may be reduced by achieving a treatment margin of any width for avascular tumors and a margin of 1 cm for vascular tumors during local ablation therapy for small HCCs of < or =3 cm.