PURPOSE: To evaluate the frequency of peritumoral bone marrow(BM) edema accompanying benign giant celltumor(GCT) of the appendicular bone by magnetic resonance(MR) imaging and to correlate MRI findings with those ofplain radiography and bone scintigraphy. MATERIALS AND METHODS: Eighteen cases of pathologically proven benignGCT of the appendicular bone were retrospectively analyzed using MR images, plain radiographs and bonescintigrams. A plain radiograph was available in 15 cases, and a scintigram in six. Marrow edema was defined asperitumoral signal changes which were of homogeneous intermediate or low signal intensity(SI) on T1WI and high SIon T2WI, relative to the SI of normal BM, and homogeneous enhancement on Gd-DTPA-enhanced T1WI. The transitionzone, sclerotic margin and aggressiveness of the lesion were assessed on the basis of plain radiographs. BM edemaseen on MR images was correlated with plain radiographic and scintigraphic findings. RESULTS: 1. Peritumoral BMedema was seen on MR images in 10 of 18 cases (55.5%). 2. In 8 of 15 cases for which plain radiographs wereavailable, MR imaging revealed BM edema. In six of these eight, transition zone was wide, while in two it wasnarrow. Six of seven patients without marrow edema showed a wide transition zone, and in one this was narrow.There was significant correlation between BM edema shown by MR imaging and the transition zone seen on plainradiographs (x2, p<0.05). But The aggressiveness shown by plain radiographs correlated only marginally, while thepresence of sclerotic rim did not correlate. 3. All six cases for which a bone scintigram was available showed anextended uptake pattern. In five of the six, MR imaging revealed edema. CONCLUSION: Peritumoral BM edema wasfrequently seen (55.5%) in the GCTs of appendicular bone ; it was more often shown in association with a widetransition zone by plain radiographs.