ABSTRACT
BACKGROUND: Epstein-Barr virus-positive mucocutaneous ulcer is one of the mature B-cell lymphoproliferative diseases occurring in patients with immune dysfunction including those with immunosuppressive treatment such as methotrexate. CASE PRESENTATION: A Japanese elderly man in his 80s with rheumatoid arthritis on methotrexate was admitted to our hospital complaining persistent pharyngeal pain. Laboratory tests revealed severe pancytopenia, elevated C-reactive protein, and increased creatinine levels. An otolaryngological examination showed ulceration of the right tonsil, from which diagnostic biopsy was performed. The diagnosis of Epstein-Barr virus-positive mucocutaneous ulcer was made and bone marrow aspiration revealed hypocellularity and megaloblastic changes. Pancytopenia was improved after discontinuing methotrexate, and repeated bone marrow aspiration test revealed recovery of normal cellularity and disappearance of dysplasia, confirming the diagnosis of methotrexate intoxication. Tonsil ulcer was improved only with discontinuation of methotrexate, which strongly supported the diagnosis of EBV-MCU. CONCLUSION: Our case suggested that even this best prognosis form of lymphoproliferative disease could lead to fatal complications if not appropriately managed.