Patients receiving methotrexate (MTX) for the treatment of autoimmune disease are at a high risk of developing lymphoproliferative disorders (LPD), the so-called methotrexate-associated lymphoproliferative disorders (MTX-LPD). We recently performed abdominal ultrasonography (US) in a patient with rheumatoid arthritis (RA) who had developed hepatic dysfunction during the course of MTX therapy; the examination revealed multiple well-demarcated hepatic tumors with slightly irregular borders, the largest one measuring 9 cm in diameter. In view of the finding of portal and hepatic veins perforating the tumor, we suspected a diagnosis of malignant lymphoma and performed a hepatic tumor biopsy. Histopathological examination of the biopsy specimens revealed a diagnosis of diffuse large B-cell lymphoma, and we made a final diagnosis of MTX-LPD. MTX treatment was discontinued, which resulted in rapid resolution of the lesions. Resolution of MTX-LPD can be obtained just by discontinuation of MTX treatment. In patients receiving MTX therapy who are found to have hepatic tumors perforated by the portal vein and/or hepatic vein on abdominal US, it is advisable to perform hepatic tumor biopsy to facilitate differential diagnosis of MTX-LPD and enable a definite diagnosis.