© 2021 Elsevier España, S.L.U. and Sociedad Española de Reumatología y Colegio Mexicano de ReumatologíaIntroduction and objectives: The etiopathogenesis of ankylosing spondylitis (AS), which is a chronic, progressive, inflammatory, systemic disease, has not been fully elucidated yet. Thiol-disulfide homeostasis, a component of antioxidant defense, is thought to play a role in the etiology of inflammatory diseases. We aimed to evaluate the existence of oxidative stress in active AS patients with thiol-disulfide homeostasis. Materials and methods: Patients who were found to have high (n: 27) and very-high (n: 18) activity levels with ASDAS-ESR and 40 healthy controls participated in the study. Serum native-thiol (NT), total-thiol (TT), and disulfide levels were analyzed by an automated colorimetric method. Results: While TT and NT levels were significantly decreased in patients compared to the control group, the disulfide levels were increased. There was a significant negative correlation between ESR, and NT, TT in both groups and also between hsCRP and NT, TT in very-high active AS patients.TT and NT levels were significantly higher in the nonsteroidal anti-inflammatory drug (NSAID) users compared to those using biological agents. Conclusions: The deterioration of thiol-disulfide homeostasis in favor of disulfide; correlations between ESR, CRP, and NT, TT support the use of thiol-disulfide variables in determining the disease activity level.