The pathogenesis of multiple sclerosis (MS), the major neurological disease of young adults in the western world, is still poorly understood and no effective therapy to block MS is yet available. It is generally accepted that reactive oxygen species have a major role in the mediation of cell damage and that free sulfhydryl groups are vital in cellular defense against endogenous or exogenous oxidants. Modification of the cellular oxidant/antioxidant balance has been involved in the neuropathogenesis of several diseases, e.g., stroke, Parkinson's disease, Alzheimer's disease and physiological aging. An increasingly important area of antioxidant defense is based on sulfhydryl chemistry, owing to the role of sulfhydryl groups in the function of macromolecular structures such as enzymes and cellular membranes. The chemical composition of human cerebrospinal fluid (CSF) is considered to reflect brain metabolism and in the present study we provided experimental evidence of a decrease in sulfhydryl groups and increased content of products of lipid peroxidation, such as ultraweak chemiluminescence and liposoluble fluorescence, which we found higher in the CSF and plasma of MS patients than in controls, pointing out the role of oxidative stress in the pathogenesis of MS.