A 43-year-old woman was admitted with a tumor mass in her forehead. Two months previously, a lump in her breast had been diagnosed as mastopathy. Palpation revealed an elastically hard immobile tumor mass in her forehead. MRI detected a tumoral lesion of generally uniform contrast involving frontal subcutaneous, cranial, and intracranial regions. PET demonstrated more intensive and wider accumulation of [11C]methyl-L-methionine (Met) than of [18F]fluoro-2-deoxyglucose (FDG). Biopsy of the forehead mass was performed, which was diagnosed as B-cell-type malignant lymphoma. The tumor mass in the forehead then shrank spontaneously, as confirmed by palpation and MRI. The tumor mass in the left breast was totally extirpated and histologically diagnosed as B-cell-type malignant lymphoma, like the tumor mass in the forehead. Postoperatively, chemotherapy (VEPA) was performed. Although FDG accumulation had not been detected, postchemotherapy PET demonstrated slight Met accumulation, suggesting the presence of a residual tumor. PET served well to detect the lesion and evaluate therapeutic efficacy in malignant lymphoma. Met-PET was more sensitive to malignant lymphoma than FDG-PET.