The assessment of residual tumors after treatment of malignant lymphoma (ML) is often difficult. Here we report a case of non-Hodgkin's lymphoma with a huge sacral tumor. After chemotherapy and following radiation therapy, a residual mass was detected on magnetic resonance imaging (MRI). However, a hypermetabolic lesion in the sacrum disappeared on (18)F-fluorodeoxyglucose positron emission tomography ((18)F-FDG-PET) and clinically the patient was considered to achieve complete remission. Seven months after the completion of radiation therapy, a new tumor-like lesion in the sacrum developed on MRI, but hypermetabolic lesions were not detected on (18)F-FDG-PET. Recurrence of lymphoma was denied by open biopsy of the lesion. (18)F-FDG-PET has been of widespread use not only for staging but for post-treatment assessment of ML. Although MRI is a standard imaging tool for the assessment of bone involvement of ML, there have been few reports documenting the results of comparative studies on the usefulness of (18)F-FDG-PET and MRI for the evaluation of residual mass in bone involvement of ML. The present case suggests that (18)F-FDG-PET is superior to MRI not only in the evaluation of a residual mass but in the judgment of recurrence after treatment of such patients.