We reported a 13-year-old boy diagnosed as multiple sclerosis associated with narcolepsy. He had suffered from retrobulbar optic neuritis at the age of 11 years which was improved gradually by prednisolone. Four months later he was admitted because of excessive somnolence. The diagnosis of narcolepsy was made based on hypnagogic hallucination, sleep paralysis, changes of personality and the sleep onset sREM (SOREM). The elevation of anti-measles antibody and a positive oligoclonal band in CSF, low density areas in the bilateral internal capsule on CT scan, and high signal areas in the same region on T2-weighted MRI confirmed the diagnosis of multiple sclerosis. An abnormal arousal response occurred occasionally in sleep stage 2 and 4, which started with electrical silence followed by a period with irregular high voltage slow waves and repetitive focal spike train. Those clinical symptoms and abnormalities of biochemical and electrophysiological studies normalized after treatment with prednisolone. However, abnormalities on MRI showed no improvement even after long term administration of prednisolone (2.5 mg/day).