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Background/Aims: Biologics are very effective drugs for patients with ankylosing spondylitis (AS). However, there are patients who are not responding to biologics. This study aimed to evaluate the level of tumor necrosis factor α (TNF-α), inter leukin (IL)-23, and IL-17 from synovial fluid in patients with AS and rheumatoid arthritis (RA) and differences of the level of those cytokines according to drugs. Methods: Synovial fluid was obtained from 34 patients (42 samples) with AS and 45 patients (47 samples) with RA with active arthritis of the knee, and the cytokine levels were measured. The differences in the levels between patients treated with and without biologics (biologics and non-biologics groups, respectively) were an alyzed in AS and RA. The correlations between cytokines were examined in the non-biologics and biologics groups. Results: The TNF-α level in AS was significantly lower than that in RA (p = 0.016). The IL-17 and IL-23 levels were not different between AS and RA (p = 0.409 and p = 0.562, respectively). In AS and RA, TNF-α, IL-17, and IL-23 showed good correla tion among each other in the non-biologics group. However, there was no signifi cant correlation in biologics group. In some patients in the AS group, the IL-17 or IL-23 level was markedly elevated in the biologics group. Conclusions: Treatment with biologics affects the cytokine profile in inflamma tory synovial fluid in patients with both AS and RA. Furthermore, IL-23 and IL-17 cytokine might be an important factor in some patients who are unresponsive to biologics in AS.