Article abstract-We retrospectively reviewed the clinical records of all patients with cancer and brachial plexopathy who had an MRI of the brachial plexus between 1984 and 1993 (71 patients). The MRIs were reevaluated in a blinded fashion. The presence of a mass adjacent to the brachial plexus on MRI was highly predictive of tumor infiltration as determined by clinicopathologic criteria and was the most useful feature in distinguishing radiation plexopathy from tumor infiltration. Increased T2 signal in or near the brachial plexus was commonly seen in both groups and was not useful in this distinction. MRI was very sensitive for brachial plexus abnormalities in this condition, and limited comparison with CT suggested that MRI is superior to CT as an imaging modality. CT performed sufficiently well, however, to suggest that a prospective comparison study of the cost effectiveness and clinical utility of the two imaging modalities in this clinical setting is warranted.