Serum selenium concentration, glutathione peroxidase activity and lipid peroxides were determined in patients with multiple sclerosis (MS). The series consisted of 13 same-sexed twin pairs derived from the Finnish Twin Cohort of 15,815 pairs. Fourteen subjects had a definite and 1 a probable MS, and their 11 co-twins showed no evidence of central nervous system disease. No statistical differences were observed, but the 3 patients with active progressive MS had a higher mean level of lipid peroxides than the rest of the patients. We suggest that serum lipid peroxidation may be involved in the activity of MS.