Background Familial Mediterranean fever (FMF) is a disease having inflammatory attacks.1 Systemic inflammation has an important role in the etiology of endothelial dysfunction. Fibroblast Growth Factor 23 (FGF-23) is expressed by osteoblasts and needs serum soluble klotho. FGF-23 provides phosphate regulation. Increased FGF-23 is associated endothelial dysfunction.2 An invasive way of measuring endothelial dysfunction is Flow-Mediated dilatation (FMD).3 Objectives To investigate the possible relationship between FGF23, serum soluble klotho levels and FMD in patients with FMF and healthy subjects. Methods Between March 2017 - September 2017, sixty -FMF patients that following-up at Cumhuriyet University Medical Faculty Rheumatology-Internal Medicine Department and thirty healthy volunteers were included into the study. Blood samples were taken from all participants and serum soluble klotho, FGF23 values were measured. Clinical findings of all patients were recorded. Blood tests were examined by Elisa method in Cumhuriyet University Department of Biochemistry. FMD assessed by Doppler ultrasound in all participants. Results Mean serum FGF 23 level was measured as 221.01 pg/ml pg in FMF group and 99.4 pg/ml in healthy control (HC) group. There was statistically significant difference between two groups (p<0.05). The mean serum levels of serum soluble Klotho was measured as 3.0 pg/ml FMF group and 8.25 pg/ml in the HC group. There was statistically significant difference between the two groups (p<0.05) (figure 1). The mean percentage of FMD was measured as 10.1 in FMF group and 13.6 in the HC group. There was statistically significant difference between the two groups (p<0.05).Abstract THU0608 – Figure 1 Serum Levels of FGF-23 and Serum Soluble klotho in FMF patients and control group Conclusions We found that high FGF 23 levels in FMF patients caused a significant decrease in FMD. In addition, FGF23 may be a parameter for the early diagnosis and prognosis of possible cardiovascular events showing significant change in FMD which demonstrates endothelial dysfunction indirectly. References [1] Sahin A, Karakus S, Durmaz Y, et al. Evaluation of Ovarian Reserve with Anti-Müllerian Hormone in Familial Mediterranean Fever. Int J Rheumatol2015;2015:380354. doi:10.1155/2015/380354. Epub 2015 May 12 [2] Silswal N, Touchberry CD, Daniel DR, et al. FGF23 directly impairs endothelium-dependent vasorelaxation by increasing superoxide levels and reducing nitric oxide bioavailability. Am J Physiol Endocrinol Metab2014Sep 1;307(5). [3] Ghiadoni L, Salvetti M, Muiesan ML, Taddei S. Evaluation of endothelial function by flow mediated dilation: methodological issues and clinical importance. High Blood Press Cardiovasc Prev2015Mar;22(1):17–22. Acknowledgements: We would like to thank to ”Cumhuriyet University Scientifical Projects Unit (CUBAP)” for funding this project. Disclosure of Interest None declared