An increased risk of lymphoproliferative disorders (LPD) has been demonstrated in patients treated with methotrexate (MTX) for rheumatoid arthritis (RA). The role of Epstein-Barr virus (EBV) has been discussed in the pathogenesis of immunodeficiency-associated LPDs. We herein present a RA patient, who developed Burkitt lymphoma during MTX treatment. The patient was a 61-year-old Japanese female with a 10-year history of weekly MTX therapy for RA. She presented with a one-month history of submandibular lymph node swelling and fever. Remarkable increases in serum lactate dehydrogenase and blood EBV DNA were observed. Serology for HIV was negative. Biopsy specimens demonstrated diffuse proliferation of medium-sized lymphoid cells. The cells were positive for CD10, CD20 and BCL6, and negative for BCL2, MUM1, terminal deoxynucleotidyl transferase and CD34. The MIB-1 index was almost 100%. EBV in the tumor cells was identified by using EBV-encoded RNA in situ hybridization. A chromosomal translocation t(8;14) was found and further confirmed by fluorescence in situ hybridization. Her condition improved following discontinuation of MTX and initiation of prednisolone. After three cycles of a dose-reduced CHOP-like regimen, chemotherapy was discontinued due to severe complications. However, there has been no sign of recurrence for six years to date without additional intensive chemotherapy.