The aim of this study was to investigate serum paraoxonase and arylesterase activities, and to determine oxidative status via the measurement of total oxidant status (TOS), total antioxidant status (TAS) and the oxidative stress index (OSI) in patients with relapsing-remitting multiple sclerosis (RRMS). Results were compared with data from healthy controls. A total of 60 subjects, including 30 newly diagnosed and untreated patients with RRMS (20 females, 10 males, 18-40 years of age) and 30 healthy controls (20 female, 10 male 20-40 years of age) were enrolled in this study. The oxidative status of the RRMS patients was measured by TOS, TAS and estimation of the OSI was made by a new automated method. Paraoxonase (PON1) and arylesterase activities were measured spectrophotometrically. TAS levels of RRMS patients were significantly lower than that of controls (p < 0.05). TOS levels of RRMS patients were higher than that of controls (p < 0.05). PON1 and arylesterase activities of RRMS patients were lower, but not significantly, than those of controls (p > 0.05). There was no correlation between serum PON1 activity and OSI in patients with RRMS (p > 0.05). Hypercholesterolemia was not observed in multiple sclerosis patients. In conclusion, although the mechanism underlying the significant reduction of TAS levels of multiple sclerosis patients compared with those of controls is unknown, the results imply that endogenous antioxidants may have been exhausted by increased oxidative stress and we believe that additional antioxidant treatment might be beneficial for these patients. Crown Copyright (C) 2012 Published by Elsevier Ltd. All rights reserved.