Aim: The present study aims to compare interleukin 6 concentration in synovial fluid in patients with known types of arthritis.Background: Persistent synovitis without known markers, such as Rheumatoid Factor (RF), Anti- Citrullinated Protein Antibodies (ACPA), and genetic markers as HLA-B27, is not uncommon. It is valuable to determine the presence of chronic inflammation and put it in correlation with agerelated changes, which are especially relevant for middle-aged patients with mono- or oligoarthritis, when the dilemma to start disease-modifying drugs for inflammatory disease often is present. Interleukin 6 (IL-6) plays a significant role in chronic inflammation.Objectives: IL-6 concentration in synovial fluid reflects the presence and activity of joint inflammation.Methods: Synovial fluid was obtained from 101 patients with chronic synovitis. IL-6 concentration was determined by the immunochemical luminescence method.Results: The median IL-6 concentration in synovial fluid in patients with osteoarthritis (OA) was 138.0 pg/ml (interquartile range (IQR) 43.4 to 296.0); in patients with rheumatoid arthritis was 2516.5 pg/ml, (IQR 1136.0 to 25058.0); in reactive arthritis 2281.0 pg/ml (IQR 1392.0 to 8652.0); psoriatic arthritis 1964.0 pg/ml (IQR 754.0 to 7300.0); ankylosing spondylitis 2776.0 pg/ml (IQR 514.7, 3944.0); in a group with negative RF, ACPA and HLA-B27 inflammatory arthritis 2163.0 pg/ml (IQR 822.0 to 7875.0). There is statistically significant difference of IL-6 concentration comparing OA and each inflammatory arthritis group, p<0.0001.Conclusion: IL-6 detection in the synovial fluid is helpful in arthritis evaluation. The results show that an IL-6 level over 1000 pg/ml suggests the diagnosis of inflammatory arthritis.