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Rheumatoid arthritis (RA) is a chronic inflammatory condition characterized by synovial joint inflammation. It leads to bone loss and painful cartilage breakdown.YKL-40 is a heparin-human cartilage glycoprotein-39 with a molecular weight of 40 kDa that is secreted by some cell types in the arthritic joint but has no enzymatic action.YKL-40 may be a marker for inflammation, tissue remodeling, or degradation. In this study, we evaluated YKL-40 in RA patients and compared them to the control group, as well as the relationship with disease activity score (DAS-28)and Anti-Cyclic citrullinated peptide antibody (ACCPAb)The study design was based on the Case-control study methodology, with 45 patients with Rheumatoid arthritis the diagnosis made in by specialist consultation at the Rheumatoid Unit at Baghdad Teaching Hospital of Iraq and 45 healthy controls.Disease activity score 28 (DAS-28) was calculated. The level of serum YKL-40 and serum Anti-ccpAb were measured using an enzyme-linked immunosorbent assay (ELISA). RA patients had considerably higher serum YKL-40 levels (157.62 62.13 ng/ml) than normal controls (124.51 30.72 ng/ml).( p,0.001). Furthermore, anti-ccpAb levels in RA patients were considerably higher ( 232.37 U/ml ) than in normal controls (4.76 2.04 U/ml). (p=0.000).There were no significant correlations between serum YKL-40 levels and anti-cyclic citrullinated peptide ( r,-0.056, P,0.714 ),and ESR ( r,0.008, P,0.957 ),RF(r, 0.086, P,0.655),CRP(r,0.074, P,0.576).There was significant differentiability in serum YKL-40 levels between remission and server DAS-28 disease activity scores. (p,0.02).on the other hand, there was no significant difference in serum YKL-40 levels among RA patients who were treated with groups DMARDs (p,0.919). Additionally, The level of serum YKL-40 was shown to have a strong positive relationship with disease activity.(r,0.345, P,0.020). In comparison to normal controls, RA patients had elevated serum YKL-40 levels, which were positively associated with disease activity and showed a large variation in serum YKL-40 levels due to disease activity. As a result, YKL-40 is a new biomarker that can be used to measure disease activity in RA patients.