Malignant lymphoma associated with rheumatoid arthritis, developing shortly after initiation of oral methotrexate.

We report a patient with rheumatoid arthritis (RA) who developed malignant lymphoma of the diffuse large B-cell type in the right submandibular region shortly after initiation of oral methotrexate (MTX). Despite cessation of MTX, the lymphadenopathy did not regress, and only reached complete remission after 3 courses of CHOP therapy followed by irradiation. In this patient highly active RA itself was considered to be the main cause of malignant lymphoma, and MTX might have contributed to the development by modifying the immune system. When RA is highly active, MTX should be used carefully because of the possible development of malignant lymphoma as well as other serious complications.