The role of leptin was investigated in relapsing-remitting multiple sclerosis (MS). Control and MS patients showed comparable baseline serum leptin levels. During the first year of IFNbeta-1a treatment, leptin significantly decreased since 2 months after starting therapy in 11 patients who had no relapses. A significant decrease in IL12/IL10 ratio was observed in this group of patients only after 1 year of treatment. An increase of leptin was observed before the first clinical exacerbation in 13 relapsing patients. Leptin may play a pathogenic role in MS and can be a useful marker of disease activity and response to therapy.