A 58-year-old Japanese woman complained of a painful right maxillary premolar gingiva and ulcer. The patient had RA and had been treated with several immunosuppressive drugs such as methotrexate. Head and neck CT indicated no obvious bone destruction with maxillary. However, chest CT indicated the presence of nodular mass of the bilateral lungs. FDG-PET/CT indicated the presence of increased uptake in both lesions. On immunohistochemistry, atypical large-sized lymphocytes were positive for CD20 and EBER-ISH and negative for CD3, CD5, and CD10; the Ki-67 labeling index was high, the histopathological diagnosis was EBV-positive DLBCL, and the clinical diagnosis was MTX-LPD. The patient's treatment with MTX was then discontinued; we removed the alveolar bone which necrosed after 5 weeks. The lesion and the nodular mass at the bilateral lungs had completely disappeared after 7 weeks.