Primary hepatic lymphoma (PHL) is an extremely rare disease, frequently associated with viruses such as hepatitis B virus (HBV), hepatitis C virus (HCV), and human immune deficiency virus (HIV). On the other hand, an increased risk of lymphoproliferative disorders (LPD) has been demonstrated in patients treated with immunosuppressive drugs such as methotrexate (MTX) for rheumatoid arthritis (RA). The role of Epstein-Barr virus (EBV) has been discussed in the pathogenesis of the immunodeficiency-associated LPDs. We here describe a RA patient, who developed PHL during RA treatment. The patient was a 64 year-old Japanese male with a 2-year history of RA, who had been treated with MTX at weekly dose of 8-14 mg for 2 years and infliximab (IFX) for 7 months. He presented with a 2 month history of generalized malaise, right hypochondrium pain and fever. Contrast-enhanced computed tomography (CECT) of the abdomen showed multiple irregular and nodular liver masses with a maximum of 13 cm in diameter on the right liver. Biopsy specimens demonstrated CD20-positve diffuse large B-cell lymphoma (DLBCL), but EBV was not identified by EBV-encoded RNA in situ hybridization. Serology for HBV, HCV, human T-cell leukemia virus I (HTLV-I), and HIV was negative. His symptoms disappeared following discontinuation of RA treatment including MTX. A drastic regression of the tumor masses was further obtained without cytotoxic chemotherapy. In addition, although the patient had no past history of liver dysfunction before MTX therapy, persistent elevation of liver enzymes has been observed during MTX treatment. These findings show a causative role of MTX in the development of reversible PHL in the patient.