Multiple sclerosis (MS) patients were found to have elevated thiobarbituric acid reactive material levels, increased soluble sulfhydryl groups and reduced protein sulfhydryl groups in cerebrospinal fluid and serum, and slightly reduced superoxide dismutase in serum, which suggested disease activating free radical peroxidation. Moreover, levels of these varied across methylprednisolone (MP) therapy. We observed significant differences in the levels of peroxidation products between MS patients and controls. These changes were most evident in relapse. After MP therapy, levels of these indicators approached control values, especially in the remission period. Our findings suggest that MP protects against free radical attack.