We report a case that fluorodeoxyglucose positron emission tomography (FDG PET)/computed tomography (CT) depicted systemically multiple lesions of methotrexate (MTX)-associated lymphoproliferative disorders. A 70-year-old man receiving MTX for rheumatoid arthritis complained of neck swelling. FDG PET/CT revealed multiple FDG-avid lesions in lymph nodes, lungs, bones, and muscles. Lesions in bones and muscles were not detected on X-CT. Final diagnosis of cervical node was confirmed as MTX-associated lymphoproliferative disorders. FDG PET/CT performed 148 days after discontinuing MTX demonstrated complete remission.