Methotrexate is effective in treating rheumatoid arthritis (RA). Some reports have discussed the possible association between methotrexate and lymphoma. Here, we report a case of pulmonary non-Hodgkin's lymphoma (NHL) developed after 11 years' methotrexate therapy for RA. Biopsy of the pulmonary mass demonstrated a diffuse large B-cell lymphoma. After withdrawal of methotrexate without any other intervention for 4 weeks, a significant reduction in the size of the lymphoma was observed. The causative relationship between methotrexate and pulmonary lymphoma is suggested by the persistent remission after stopping methotrexate therapy.