A 6-year-old female patient with acute disseminated encephalomyelitis associated with poliomyelitis vaccine virus is reported. She had a history of high fever, headache, and gait disturbance. Neurologic examination confirmed spastic triparesis, urinary incontinence, diminution of tactile sensation, and vision deterioration. Hemography, serum laboratory findings, and urinalysis were normal. The cerebrospinal fluid was clear, with normal pressure, 9 leukocytes/mm(3), and 27 mg/dL protein, but the myelin basic protein was elevated to 10.7 ng/mL. T(2)-weighted magnetic resonance imaging disclosed multifocal high-intensity lesions of the spinal cord. The serum polio virus type 2 antibody titer was raised in the acute phase, and polio vaccine virus type 2 was detected in viral cultures of the cerebrospinal fluid and pharynx swab and had undergone an A-G neurovirulence mutation at nucleotide 481. Finally, she had human leukocyte antigen (HLA)-Cw3 and HLA-DR2, to which multiple sclerosis is related in Japan. Thus the cause of ADEM may have been related to her HLA type.