['Dickkopf-1 (DKK-1) has been proved dysfunctional in ankylosing spondylitis (AS), and decreased serum functional DKK-1 levels have been identified to be decreased in AS patients. This study aims to examine the relation of binding (functional) DKK-1 levels in serum with the radiographic severity of AS.', 'Total Dkk1 levels were assessed by commercial sandwich enzyme-linked immunosorbent assay and functional DKK-1 levels of serum were measured using functional enzyme-linked immunosorbent assay method in 84 patients with AS and 79 healthy controls. The radiographic progression of AS was classified according to the radiographic events of modified New York Criteria for sacroiliac joints evaluation and modified Stoke Ankylosing Spondylitis Spine Score (mSASSS) system for spine assessment.', 'No significant difference was seen between AS patients and healthy controls for total Dkk1 levels (p = 0.273). Decreased levels of functional DKK-1 in serum were found in AS patients compared with healthy controls 17.24 (1.24-11.15) vs. 9.15 (5.33-13.35) pg/mL, p < 0.001]. Functional DKK-1 levels in serum of AS patients were significantly associated with the disease radiographic severity evaluated by modified New York grading criteria (r = -0.236, p = 0.04) and mSASSS system (r = -0.389, p = 0.025).'] Are serum functional dickkopf-1 levels inversely correlated with radiographic severity of ankylosing spondylitis?