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A patient with rheumatoid arthritis (RA) who was being treated with methotrexate (MTX) therapy presented with severe swelling of the left elbow. Magnetic resonance imaging showed a tumor-like lesion around the elbow joint. Fluorodeoxyglucose positron emission tomography indicated multiple lesions in the lung and the lymph nodes. An open biopsy of a cervical lymph node was performed, and MTX-related lymphoproliferative disorder (MTX-LPD) was diagnosed. After cessation of the MTX therapy, the elbow swelling regressed, and the patient was in remission of MTX-LPD.MTX-LPD should be considered in the differential diagnosis when a patient with RA develops severe joint swelling while on MTX therapy.