To compare the change in nitric oxide (NO) metabolism in patients with multiple sclerosis (MS) in different clinical stages, such as exacerbation, remission, and 1-2 months after methylpredonisolone administration, we measured the nitrite and nitrate (NO2-/NO3-) levels in the cerebrospinal fluid (CSF). In the extreme stage of MS exacerbation, those levels were remarkably elevated, compared with those both after methylpredonisolone administration and remission. These results suggest that, in addition to MRI study, measurement of NO metabolites in the CSF may give information to help us understand the activity of MS and effect of drugs on the disease.