To evaluate the features on magnetic resonance (MR) images of malignant and nonmalignant retroperitoneal fibrosis (RPF) and to assess the usefulness of MR imaging in differentiating the two conditions, MR studies of nine patients with malignant RPF were retrospectively examined and compared with those of eight patients with nonmalignant RPF. Morphologic findings at MR imaging were similar for both conditions. The lesions were, however, heterogeneous in six of the nine patients with malignant RPF and homogeneous in all eight patients with nonmalignant RPF. On T2-predominant images, malignant RPF showed high signal intensity, while nonmalignant RPF showed low signal intensity. On T2-predominant images, signal intensity and T2 were significantly higher in malignant RPF than in nonmalignant RPF. Differentiation between malignant and nonmalignant RPF appears feasible and depends on tissue contrast rather than on morphologic characteristics.