Background:Vasovagal syncope (VVS)is common in children and greatly affect both physical and mental health.But the mechanisms have not been completely explained.This study was designed to analyze the gut microbiota in children with VVS and explore its clinical significance.Methods:Fecal samples from 20 WS children and 20 matched controls were collected,and the microbiota were analyzed by 16S rRNA gene sequencing.The diversity and microbiota compositions of the WS cases and controls were compared with the independent sample t test or Mann-Whitney U test.The correlation between the predominant bacteria and clinical symptoms was analyzed using Pearson or Spearman correlation test.Results:No significant differences in diversity were evident between VVS and controls (P ＞ 0.05).At the family level,the relative abundance of Ruminococcaceae was significantly higher in WS children than in controls (median [Q1,Q3]:22.10％ [16.89％,27.36％] vs.13.92％ [10.31％,20.18％],Z=-2.40,P＜ 0.05),and LEfSe analysis revealed Ruminococcaceae as a discriminative feature (linear discriminant analysis [LDA] score ＞ 4,P ＜ 0.05).The reladve abundance of Ruminococcaceae in WS patients was positively correlated with the frequency of syncope (r =0.616,P ＜ 0.01).In terms of its correlation with hemodynamics,we showed that relative abundance of Rumainococcaceae was negatively correlated with the systolic and diastolic pressure reduction at the positive response in head-up tilt test (HUTT;r=-0.489 and-0.448,all P ＜ 0.05),but was positively correlated with the mean pressure drop and decline rate (r=0.489 and 0.467,all P＜ 0.05) as well as diastolic pressure drop and decline rate at the HUTT positive response (r=0.579 and 0.589,all P＜0.01) in WS patients.Condusion:Ruminococcaceae was the predominant gut bacteria and was associated with the clinical symptoms and hemodynamics of WS,suggesting that gut microbiota might be involved in the development of WS.