Multiple sclerosis (MS) is an autoimmune disease of the central nervous system characterized by myelin breakdown. The free radical nitric oxide (NO), which is considered to be a major metabolite in immune function and in autoimmune disorders, is among the possible mediators causing the inflammatory reactions in MS. Consequently, NO has been implicated in the pathogenesis of MS and its animal model experimental allergic encephalomyelitis (EAE). In this study, stable metabolites of NO (NO2- + NO3-) levels were determined in sera of MS patients (n = 23) and control subjects (n = 16). NO2- + NO3- levels were higher in MS patients when compared to control subjects. However, there was not any correlation with serum NO2- + NO3- values and clinical features of the disease such as duration of sickness, the time elapsed from the last attack and EDSS values. Our results imply that nitric oxide may be involved in the pathogenesis of MS although further studies are required to elucidate underlying mechanisms. © 2002 Published by Elsevier Science Ltd.