Background: The role of oxidative stress in patients with multiple sclerosis (MS) is poorly understood. Objective: To investigate oxidative stress in serum and peripheral blood leukocytes in patients with different disease courses of MS. Methods: Diene conjugate (DC) levels (a measure of lipid peroxidation), total antioxidative activity (AOA) and total antiradical activity (ARA) were measured in serum and peripheral blood leukocytes from 30 patients with benign relapsing remitting MS (BMS), 27 with secondary progressive MS (SPMS), 29 with primary progressive MS (PPMS), and 30 healthy controls. All MS patients were in a clinically stable phase. Results: Serum DC levels were elevated in patients with BMS (p < 0.05), SPMS (p < 0.01) and PPMS (p < 0.001). Serum total AOA and ARA were not different between MS patients and controls. Compared to controls, leukocyte DC levels were not different in each MS subgroup, but total ARA was elevated. There was a strong correlation, both in controls and MS patients, between leukocyte DC levels and leukocyte total ARA (p < 0.0001) and leukocyte total AOA (p < 0.0001). Conclusion: Oxidative stress occurs in progressive as well as benign MS. The finding that cells withstand oxidative stress, due to upregulated cellular antioxidant defence mechanisms, suggests that reactive oxygen species (ROS) formation in MS is not necessarily deleterious. Background: The role of oxidative stress in patients with multiple sclerosis (MS) is poorly understood. Objective: To investigate oxidative stress in serum and peripheral blood leukocytes in patients with different disease courses of MS. Methods: Diene conjugate (DC) levels measure of lipid peroxidation, total antioxidant activity (AOA) and total antiradical activity (ARA) were measured in serum and peripheral blood leukocytes from 30 patients with benign relapsing remitting MS (BMS), 27 with secondary progressive MS (SPMS) Primary progressive MS (PPMS), and 30 healthy controls. All MS patients were in a clinically stable phase. Results: Serum DC levels were elevated in patients with BMS (p <0.05), SPMS (p <0.01) and PPMS Compared to MS patients and controls. Compared to controls, Leukocyte DC levels were not different in each MS subgroup, but total ARA was elevated. There was a strong correlation, both in controls an d MS patients, between leukocyte DC levels and leukocyte total ARA (p <0.0001) and leukocyte total AOA (p <0.0001). Conclusion: Oxidative stress occurs in progressive as well as benign MS. The finding that cells withstand oxidative stress, due to suggestsregulated cellular antioxidant defense mechanisms, suggests that reactive oxygen species (ROS) formation in MS is not necessarily deleterious.