Background: Knowledge on endothelial dysfunction and its relation to atherosclerosis in mastocytosis is limited. Aim: To investigate the endothelial function in mastocytosis by flow mediated dilatation (FMD) and biomarkers related to vascular endothelia, the presence of subclinical atherosclerosis by carotid intima media thickness (CIMT). Method: Forty-nine patients with mastocytosis and 25 healthy controls (HCs) were included. FMD and CIMT during transthoracic echocardiography, biomarkers including endocan, endothelin-1 (ET-1), vascular endothelial growth factor (VEGF) were measured in sera of participants. Tumor necrosis factor-alpha (TNF-α), interleukine-6 (IL-6) and high sensitive c-reactive protein (hsCRP) were determined as inflammatory biomarkers. Result: The mean FMD% was lower in the patients than HCs (11.26±5.85% vs 17.84±5.27% p<0.001) and was the lowest in the advSM and SSM group among the patients (p=0.03). The median value of VEGF was significantly higher in patients than HCs. [73.30 pg/mL; min-max (32.46-295.29) pg/mL vs (46.64 pg/mL; min-max 11.09-99.86 pg/mL; p:0.001] and it was the highest in the advSM and SSM group (p:0.01). FMD was inversely correlated with endocan (r:-.390, p:0.006), ET-1 (r:-.363, p:0.01) and VEGF (r:-.402, p:0.004) but there were no correlations between FMD and TNF-α, IL-6, and hsCRP. No differences in CIMT values between patients and HCs and no correlation between CIMT and the biomarkers were observed. Conclusion: Endothelial dysfunction in mastocytosis becomes evident with decreased FMD and elevated serum VEGF, in the absence of atherosclerosis or systemic inflammation and is related to disease severity. Keywords: CIMT, Endocan, Endothelial function, Endothelin-1, FMD, VEGF