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Objectives: The aim of this study was to investigate the association of fibromyalgia (FM) syndrome with ankylosing spondylitis (AS) and to compare the AS patients with and without FM according to the disease activity, clinical and laboratory findings, and response to treatment. Patients and methods: Between September 2016 and September 2020, a total of 511 patients (312 males, 119 females; mean age: 43.0±11.2 years; range, 18 to 77 years) who were diagnosed with AS were retrospectively analyzed. Age, sex, disease duration, disease onset age, and extra-articular findings were recorded. Medical treatments used by the patients for the treatment of AS and FM were noted. The erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), human leukocyte antigen-B27 (HLA-B27) status, and Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), Ankylosing Spondylitis Disease Activity Score (ASDAS) with ESR (ASDAS-ESR) and ASDAS-CRP values were recorded. Results: The frequency of FM in AS patients was 23.2%. Totally, 75.4% of the FM patients were female. The HLA-B27 positivity, extra-articular involvement frequency, disease duration, and acute phase reactants levels were similar between AS patients with and without FM (p=0.118, p=0.154, p=0.829, p=0.113, and p=0.763, respectively). The AS patients with FM had lower rates of achieving remission or low disease activity, compared to those without FM. The mean of all three disease activity scores between these two groups was also higher in the AS patients with FM (p<0.001). The rate of use of biological therapy was significantly higher in the AS patients with FM than those without FM (p=0.037). Conclusion: Since the treatment plan of AS is made based on the disease activity scores, unnecessary biological therapy may be initiated for patients or the biological therapies they use may be switched unnecessarily. Therefore, it should be kept in mind that FM may present with AS in patients who do not respond to treatment clinically, and this may be misinterpreted as treatment unresponsiveness. [ABSTRACT FROM AUTHOR]