Radiofrequency ablation (RFA) with a maximum output of < or = 70 W was used under CT guidance to treat 14 tumors (median diameter 1.5 cm, range 0.8 to 2.4 cm) in 6 patients. Primary lung cancer was treated in 2 patients, and metastatic lung tumors were treated in 4 patients. Good response was confirmed in 6 tumors on CT images, in one tumor on PET images, and in 2 tumors by histologic examination. In the other 5 tumors, curative effect was not determined because consolidation shadows surrounded the tumors. Complications were mild (pneumothoraces, 5; sputum cruentum, 4; subcutaneous emphysema, 3) except for one case of pleurisy. RFA of pulmonary malignancies appears to be a safe, effective treatment if a low output of less than 70 W is applied.