Objective: Our objective was to assess the relationship between disease activity and the systemic immune-inflammation index [SII; (platelet count × neutrophil count / lymphocyte count)] in individuals with ankylosing spondylitis (AS). Materials and Methods: A total of 201 participants (130 AS patients and 71 healthy volunteers) aged 18-65 years were included in this single center cross-sectional study. Based on their ratings on the Bath Ankylosing Spondylitis Disease Activity index (BASDAI), patients with AS were split into two groups: remission (n=90, those with BASDAI 4). The study employed Spearman correlation analysis to assess the relationship between SII and C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), BASDAI, Ankylosing Spondylitis Disease Activity score-ESR (ASDAS-ESR), and ASDAS-CRP. The effectiveness of SII and other measures in evaluating the disease activity in the active AS and remission AS groups was ascertained using receiver operating characteristic curve analysis. Results: SII values were significantly higher in the AS group than healthy controls, as well as in the active AS group than AS patients in remission (p