The magnetic resonance (MR) appearance of four cases of giant cell tumor (GCT) of the proximal tibia are described and the MR grading of these tumors is compared with CT and conventional radiography. Magnetic resonance showed the lesions to be well defined with respect to adjacent marrow and cortical bone. Homogeneous intermediate signal intensity or low signal within the tumors was seen on T1-weighted images. T2-weighted images showed mixed signal intensity with small "bright patches" of increased signal intensity in all four cases. No fluid levels were identified. Magnetic resonance was superior to CT and plain radiography in radiologic grading of the tumors. Computed tomography was superior in determining if cortical invasion was present. Intraarticular tumor extension was more accurately detected by MR and arthrotomography than CT. An MR manifestation of GCT of the proximal tibia is described which may be a common appearance of this tumor by this modality. Magnetic resonance is the procedure of choice in the radiologic grading of GCT.