We report here a nine-year-old girl with multiple sclerosis having consciousness disturbance at admission. Neurological examination revealed drowsiness, unstable emotion, decreased visual acuity, disturbance of convergence, and clumsy coordination movements. Her cerebrospinal fluid IgG and myelin basic protein were increased. Electroencephalogram showed intermittent, high voltage slow waves predominant in the frontal lobes. Magnetic resonance imaging (MRI) found multiple demyelinating plaques in the brainstem, thalamus, periventricular white matter. The brainstem reticular formation was involved. Since she had had bilateral acute optic neuritis and papillitis two years before the admission, the diagnosis of multiple sclerosis was made. Methylprednisolone pulse therapy improved her neurological symptoms and MRI findings. Multiple sclerosis in children, unlike that in adults, may present with symptoms mimicking an encephalopathy. Our case suggested that consciousness disturbance in childhood multiple sclerosis results from lesions in the brainstem activating reticular formation including the thalamus.