We report a 5-year-old mentally retarded Japanese boy who developed acute disseminated encephalomyelitis (ADEM) two weeks after Japanese encephalitis vaccination (Beijing strain). He presented sudden status epilepticus, fever, and disturbance of consciousness. Initial neuroradiological findings revealed multifocal cortical swellings without any white matter lesions, suggesting the existence of partial encephalitis or focal status epilepticus. On the follow-up neuroradiological examinations, small white matter lesions were identified as having gradually extended in spite of clinical improvement by methylprednisolone pulse therapy. The cortical involvement became temporarily worse along with the extension and delayed appearance of white matter lesions. Single photon emission computed tomography (SPECT) showed marked hypoperfusion of cerebral blood flow (CBF) in the cortical lesions at both the acute and the recovery period. The serial neuroradiological findings indicated involvement of white matter and gray matter regions at different stages of the illness and a delay between the onset of symptoms and the appearance of ADEM-associated MR imaging of white-matter lesions.