PURPOSE: To describe the appearance of hepatic tumors treated with radio-frequency (RF) ablation on computed tomographic (CT) and magnetic resonance (MR) images and the pattern of residual tumor at the site of RF ablation and to assess prospectively the sensitivity, specificity, and positive and negative predictive CT and MR imaging values in the evaluation of RF treatment. MATERIALS AND METHODS: Thirty-one patients with 50 tumors (nine hepatocellular carcinomas and 41 metastases) treated with RF ablation underwent CT and MR imaging on the same day at 2, 4, and 6 months; CT was performed every 3 months thereafter. CT and MR findings were interpreted separately and prospectively by two reviewers with consensus. For both imaging techniques, appearance of the treated area, treatment efficacy, and complications were assessed at each time. Sensitivity and specificity were determined by using the McNemar test. RESULTS: After a mean follow-up of 19 months, nine tumors showed local regrowth. At 2 months, MR imaging depicted more local regrowths (eight of nine; sensitivity, 89%) than did CT (four of nine; sensitivity, 44%) but without significant differences (P =.12). In two cases, only T2-weighted imaging depicted local regrowth. All nine lesions became conspicuous at 4-month follow-up with both techniques. At 2 months, thin peripheral rim enhancement and arterioportal shunting were found in 24% and 12%, respectively, of the treated tumors. These findings disappeared thereafter and are not linked to tumor regrowth. CONCLUSION: Despite the small number of patients, CT and MR imaging may depicted all local regrowth at 4 months or sooner. MR imaging may have an edge over CT in the early detection of local regrowth.