PURPOSE: This multicenter phase I/II study evaluated the safety, feasibility, and initial efficacy of radiofrequency ablation (RFA) for small malignant renal tumors. METHODS: Thirty-three patients were enrolled in the study. A single session of RFA was performed in patients with a renal tumor of 1-3 cm in greatest diameter, with the exception of lesions adjacent to the renal hilum. The primary endpoint was the safety of renal RFA, and the secondary endpoints were its feasibility and initial efficacy for local control, as well as the incidence and grade of adverse events. Clinical efficacy was evaluated by CT scans within 1 week and at a further 4 weeks after the procedure using the criteria adapted from the Response Evaluation Criteria in Solid Tumors. RESULTS: The RFA procedure was completed in 100% (95% confidence interval [CI] 89-100%) of all 33 patients. There were no severe adverse events (0% [95% CI 0-11%]). Among the 33 patients, a complete response, partial response, progressive disease, and stable disease were seen in 28 (85%), 0 (0%), one (3%), and one (3%) patient(s), respectively, with a tumor response rate of 85% [95% CI 68-95%]). Three patients (9%), including one ineligible patient (3%), were not evaluable. Out of 30 evaluable patients, a complete response was achieved in 28 (93%). CONCLUSION: The current multicenter trial revealed that RFA is a safe, feasible, and effective treatment for small malignant renal tumors in patients who are not candidates for surgery.