In six cases of giant cell tumor the magnetic resonance (MR) images obtained with various pulse sequences and field strengths were compared to the corresponding computed tomography (CT) scans and plain roentgenograms. MRI was superior to CT and plain films in demonstrating areas of tissue inhomogeneity within the tumor as well as soft tissue extension. CT was superior in demonstrating cortical thinning. Multiplanar imaging capability and visualization of articular cartilage may demonstrate intra-articular tumor spread. The characteristic MRI findings with short TR/TE (T1-weighting) and long TR/TE (T2-weighting) are described. We also describe one case where serial MR scans were used to assess response to therapy.