Considering the basic pathologic process and current knowledge of the physiopathologic mechanisms in the active phase of disseminated demyelinating disease of central nervous system, the increase of oxidative processes was assumed in the patients with that disease in the phase of clinical impairment. The aim of the research was to study some indices of oxidative processes and activity of antioxidative enzymes in such patients. The research was performed on blood samples (erythrocyte hemolysate and plasma) and cerebrospinal fluid of 30 patients with disseminated demyelinating disease of central nervous system during the clinical impairment and during the increase of immunologic activity in intrathecal space. The patients were of younger age, in the acute phase or with the shorter disease duration (up to 3 years). Control group was formed of 12 patients examined for lesions of intervertebral disks in lumbosacral region. The research results demonstrated the increase of superoxide anion production, the elevation of lipid peroxidation followed by the increase of superoxide dismutase and glutathione reductase activation. It was concluded that the signs of simultaneous increase of oxidative processes and antioxidative activity, but also the oxidative impairment of lipid structures existed in the studied patients during the acute phase.