A 66-year-old woman with a history of rheumatoid arthritis was referred to our hospital after hospitalization at a nearby general care facility for altered mental status and weakness. She had developed a parotid tumor 18 months before while undergoing disease modifying anti-rheumatoid therapy with methotrexate. The tumor was surgically removed and diagnosed as benign T-cell proliferation. The methotrexate was replaced with a monoclonal antibody at that time, and the patient developed fever, fatigue, and hepatosplenomegaly shortly thereafter. A liver biopsy again demonstrated benign T-cell proliferation despite clinical suspicion of infectious mononucleosis. The monoclonal antibody (Tocilizumab) was discontinued, and dexamethasone was administered until clinical remission. Her physician had conducted tests for malignant lymphoma and infectious mononucleosis, all of which were negative. Systemic manifestations were not detected in the radiological findings or by repeated bone marrow analyses. After a disease-free period of eight months, she started to complain of fatigue and weakness and rapidly became bedridden.