Background: To evaluate the relationship between disease activity and levels of YKL-40 in patients with Behcet's disease (BD). Methods: Twenty five active and twenty six inactive BD patients and 24 age and gender matched healthy control subjects were included in the study. Serum YKL-40 levels were measured using Enzyme Linked Immuno Sorbent Assay (ng/mL). Disease activity was assessed using Behcet's Disease Current Activity Form (BDCAF). Results: Serum YKL-40 levels were significantly higher in patients with active BD and inactive BD compared to the control group (respectively p<0.001, p< 0.001). There were no significant differences between active and inactive BD groups in terms of levels of YKL-40 (p=0.9). According to Pearson's analysis, YKL-40 levels were significantly positively correlated with Erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), and skin lesions of BDCAF scores. Conclusion: Our study demonstrated that serumYKL-40 level was significantly higher in active and inactive BD compared to healthy controls. However, the groups with active and inactive disease had no significant difference with respect to YKL-40 level. In addition, there existed a significant correlation between serum YKL-40 level and ESR, CRP, and BDCAF's skin scores.