Methotrexate (MTX) has recently been employed in the treatment of rheumatoid arthritis (RA) and has shown good therapeutic effects, but we rarely experience the development of malignant lymphoma in RA patients treated with MTX. Here we report two cases of malignant lymphoma in RA patients who were treated with MTX. Although the generating mechanism of methotrexate-related lymphoproliferative disorder (MTX-LPD) is still not fully understood, it has been hypothesised that neither the clonal multiplication of B cells induced by infection with the Epstein-Barr virus nor the polyclonal multiplication of other lymphocytes can be adequately controlled in the state of the immune suppression induced by MTX treatment of patients with an autoimmune disease. If we see RA patients who are treated with MTX, we should keep in mind that there is a risk of them developing malignant lymphoma.
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