Ankylosing spondylitis ( AS) is an in flammatory disorder with unknown etiology that mainly affects the axial skeleton as well as the peripheral joints and extra-articular structures. The aim of this study was to evaluate the total antioxidant status (TAS), total oxidative status (TOS) and oxidative stress index (OSI) in patients with ankylosing spondylitis ( AS). Fifty AS patients with a mean Bath AS Activity Index (BASDAI) 4.6 ( range 4-9.3) and 26 healthy controls were included in the study. Plasma TAS, TOS levels were determined by using novel automated methods. The OSI was calculated. Plasma TOS level and OSI values were significantly higher, and plasma TAS level was lower in patients than in healthy controls ( 15.8 +/- 4. 9 vs. 4.3 +/- 2.8, 12.8 +/- 3. 9 vs. 9.6 +/- 5.5, 1.2 +/- 0.03 vs. 1.8 +/- 0.2, respectively, P < 0.001 for all). There was no significant correlation between oxidant/antioxidant parameters and disease activity. The results of this study indicated that increased oxidant and decreased antioxidant capacity may be associated with the pathogenesis of AS.