PURPOSE: To prospectively evaluate the use of MRI with dynamic sequences during isolated limb perfusion (ILP) for soft tissue sarcomas, an aggressive local treatment using very high-dose chemotherapy and tumor necrosis factor aimed at avoiding limb amputation. DESIGN AND PATIENTS: Twenty-six patients were referred for ILP over one and a half years; eight were excluded as the lesions were either too proximal or suspicious inflammatory changes without tumor were found on the initial MRI, or the vascular status was poor. The indications for ILP were: vessel nerve involvement (13), multiple lesions (8), tumor size (4) or the presence of pulmonary metastases (2). MRI was performed 1 and 2 months after ILP, immediately prior to surgery and histological analysis. The MR examinations included T1-weighted SE and fast SE T2-weighted fat-saturated sequences, as well as dynamic sequences (T1-weighted SE repeated six times every 40 s), displaying the maximum intensity slope in each pixel. RESULTS: The tumor had disappeared in three patients. One patient still had histologically proven isolated widespread tumor cells without a mass. The tumor size had increased in two patients. In six patients, the size of the tumor had not changed but it had become completely necrotic, with a thin wall. In three patients, after an initially good result MRI demonstrated that the tumor wall had become thickened from 1 to 2 months after ILP. Dynamic MRI was mainly useful during the initial examination, demonstrating two patients with inflammatory changes without tumor. Three amputations and a second ILP were proposed based on poor results. Conservative limb-sparing surgery was successful in the other cases. CONCLUSION: MRI proved valuable in demonstrating the variable responses to ILP.