BACKGROUND The long-term use of an oral corticosteroid suppresses immunity. Here, we describe a case involving a patient with weakness in the bilateral lower extremities due to cytomegalovirus (CMV) lumbosacral polyradiculitis. METHODS A 64-year-old man visited a university hospital for symmetric motor weakness in both lower extremities (Medical Research Council grade: 2). Symptoms started 1 month before and gradually aggravated. The patient had been taking oral prednisolone for 10 years in order to control pain in multiple joints due to seronegative rheumatoid arthritis. He also had neuropathic pain on the entire right lower extremity and voiding difficulty. Gadolinium-enhanced magnetic resonance imaging revealed enhancement along the entire lumbosacral nerve roots. In the cerebrospinal fluid analysis (CSF), elevated white blood cell (WBC) count (19 cells/μL) and protein level (142.5 mg/dL) were observed. CMV detection by polymerase chain reaction (PCR) was positive. We diagnosed the patient as having lumbosacral polyradiculitis due to CMV. Ganciclovir (250 mg twice daily) was administered intravenously. Two months after initiating Ganciclovir, in the CSF analysis, CM detection by PCR was negative, and no WBC was found. CONCLUSIONS We reported a patient who had symmetric motor weakness in the bilateral lower extremities induced by CMV lumbosacral polyradiculitis. Its occurrence seems to be related to immunosuppresion due to the long-term use of an oral corticosteroid. When a patient who is taking an oral corticosteroid shows motor weakness in the bilateral lower extremities, CMV lumbosacral polyradiculitis is one of the possible disorders to be differentiated.