PURPOSE: The purpose of this study was to determine the factors influencing on the size of thermal lesions after ablation using an expandable radio-frequency needle electrode in porcine liver. MATERIALS AND METHODS: Ablation procedures involved the use of a monopolar radio-frequency generator and 15-G needle electrodes with four and seven retractable hooks (RITA Medical System, Mountain View, Cal.,U.S.A.). The ablation protocol in fresh porcine liver comprised of combinations of varying hook deployment, highest set temperature, and ablation time. Following ablation, the maximum diameter of all thermal lesions was measured on a longitudinal section of the specimen. Ten representive lesions were examined by an experienced pathologist. RESULTS: At 3-cm hook deployment of the needle electrode with four lateral hooks, the size of spherical thermal lesions increased substantially with increases in the highest set temperature and ablation time until 11 minutes. After 11 minutes lesion size remained similar, with a maximum diameter of 3.3 cm. At 2-cm hook deployment, sizes decreased to about 2/3 of those at 3 cm, and at 1-cm hook deployment lesions were oblong. At 3-cm hook deployment of a needle electrode with seven hooks, the size of thermal lesions increased with increasing ablation time until 14 minutes, and the maximum diameter was 4.1 cm. Microscopic examination showed a wide zone of degeneration and focal coagulation necrosis. CONCLUSION: The size of thermal lesions produced by the use of an expandable radio-frequency needle electrode were predictable, varying according to degree of hook deployment, highest set temperature, and ablation time.