BACKGROUND A residual mediastinal mass after treatment represents a common diagnostic problem in the management of patients with Hodgkin lymphoma (HD). Conventional diagnostic radiology, computed tomography (CT), or ultrasonography (US) do not adequately reflect changes as fibrosis or necrosis. Gallium-67 (67Ga) imaging has been proven to be useful for the evaluation of HD in the mediastinum. The authors compared the ability of gallium scan and magnetic resonance imaging (MRI) to evaluate the mediastinal disease in the follow-up of patients with HD. METHODS Thirty-four patients previously treated for HD were investigated with gallium scan, MRI, and all the other investigations to evaluate the mediastinal region. Sixteen patients were in restaging after treatment, and 18 were investigated for suspected radiologic recurrence in the mediastinum (follow-up, 9-75 months). The results of gallium scan and MRI were matched with clinical findings during the follow-up. RESULTS A sensitivity of 85.7% for 67Ga and 92.8% for MRI was found, while the specificity was 100% for the scan and 80.6% for MRI. The predictive positive value that resulted was 100% for 67Ga and 68.4% for MRI. CONCLUSIONS Both examinations were accurate in assessing the activity of residual masses in the mediastinum after treatment. 67Ga showed a lower sensitivity in comparison with MRI, but 67Ga frequently overestimates the presence of pathologic tissue. The authors acknowledge the complementary role of these two tests, but if only one of these examinations can be performed for logistic or economic reasons, then gallium scan represents the single most adequate diagnostic procedure.