Abstract: Background: Ankylosing spondylitis (AS) is a chronic systemic inflammatory rheumatic disease that specifically affects the spine and sacroiliac joint. AS diagnosis is often delayed in the clinical practice and this delay may cause the patients to miss the chance of early treatment. Fibromyalgia (FM) is a frequently encountered clinical syndrome, fibromyalgianess is a term used when patients who are diagnosed with inflammatory arthropathies meet the criteria for FM syndrome as shown in rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), Sjogren syndrome, and AS. Objectives: We aimed primarily to assess the frequency of concomitant diagnosis of FM syndrome in AS patients and study its impact on clinical disease aspects. Secondary, our aim extended as a preliminary pilot study to assess the Plasma Pentraxin-3(PTX-3) as a potential marker for the diagnosis of FM syndrome in AS patients. Methods: Plasma PTX-3 in 61 AS patients was compared to 60 matched controls. FM was diagnosed by FM Rapid Screening Tool. Bath AS disease activity index (BASDAI) and AS disease assessment score using C- reactive protein (ASDAS-CRP), Bath AS functional impairment index (BASFI), Bath AS metrology index (BASMI), AS quality of life (ASQoL) scale, Beck Depression Inventory, and Bath AS Radiology Index (BASRI) were assessed. Results: The patients were categorized into two groups according to the concomitant diagnosis of FM syndrome. Group I included 14 (22.9%) AS patients who fulfilled the clinical diagnosis of FM syndrome. Group II included 47 (77.1%) AS patients without FM syndrome. AS patients with FM (Group I) had significantly(p