Introduction and Aim: The pro-inflammatory cytokine IL-39, a member of the IL-12 family plays a key role in the inflammatory response by modulating immune cell activity and inflammation. A literature search shows no study undertaken for the effect of IL-39's on arthritis so far. Hence, the purpose of this study was to investigate the role of IL-39 in rheumatoid arthritis. Materials and Methods: This study involved 80 patients with rheumatoid arthritis registered at the Rheumatology Clinic at Baghdad teaching hospital. The patients were divided into three groups based on treatments received. Group 1 included patients who were not on any treatment for arthritis, Group 2 with patients on hydroxychloroquine and or prednisone treatment, and Group 3 that received Enbrel® (etanercept) and HUMIRA® (adalimumab) treatment for rheumatoid arthritis. A control group was included in the study. Patients in all groups were assessed for their serum IL-39 concentration, C - reactive protein, Anti-cyclic citrullinated peptide antibody (ACCP) and ESR.  Results: The patient age and BMI were not significantly different between the groups receiving treatment for Rheumatoid arthritis. A significant increase in the interleukin 39 concentration was observed in treatment groups (G1, G2, G3) as compared to normal healthy controls regardless of whether they were positive or negative for the anti-CCP test. Conclusion: This study showed that the serum interleukin IL-39 levels significantly increased in patients diagnosed with rheumatoid arthritis thus suggesting that IL-39 could be considered as a potential inflammatory biomarker of RA.