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Objective To explore the circadian rhythm of neurally mediated syncope (NMS)in children. Methods There were 21 6 children with NMS included in the study,including 91 male and 1 25 female,aged from 4 to 1 7 years old with a mean age of (1 1 .34 ±2.65)years,who came from the Specialist Syncope Outpatient Department or Inpatient Department of the Second Xiangya Hospital of Central South University from December 201 3 to October 201 5. The patients were divided into vasovagal syncope (VVS)group and postural tachycardia syndrome (POTS)group ac-cording to head -up tilt test (HUTT)results,including 1 78 VVS patients and 38 POTS patients.Ninety -four patients with NMS were in the <1 2 years old group[(8.88 ±1 .88)years old]and 1 22 subjects with NMS were in the ≥1 2 years old group[(1 3.24 ±1 .1 8)years old].All patients or guardians were carefully asked about the number of synco-pal attacks and the periods in which episodes occurred in before HUTT [24 hours of a day were divided into 4 periods:morning (0600 AM-1 200 AM),afternoon (1 200 AM-1 800 PM),evening (1 800 PM-2400 PM), night (0000 AM-0600 AM)].Results (1 )General data:the total syncopal episodes of 21 6 children with NMS were 61 4 episodes,including 1 78 VVS patients with 471 syncopal episodes in total and 1 43 attacks of 38 children with POTS.There were 273 episodes of 94 patients in the <1 2 years old group and 341 episodes of 1 22 subjects in the ≥1 2 years old group.There were no significant differences in the diurnal variation of syncopal episodes between the VVS group and POTS group regarding age and gender (P >0.05).(2)The number of syncopal episodes in patients with VVS which occurred in the morning hours was strikingly higher than that of afternoon,evening or nighttime (P <0.05).But there was no significant difference in the frequency of episodes in different periods through the day in the POTS group (P >0.05).Patients with VVS had a higher proportion of episodes in the morning but a lower proportion in the evening when compared with the POTS group (P <0.05).(3)The male children with NMS tended to have a higher proportion of episodes in the morning than the female patients(χ2 =1 1 .001 ,P =0.01 2).(4)There seemed to be no difference in the frequency of syncopal episodes through the day between the <1 2 years old group and the ≥1 2 years old group(χ2 =1 .995,P =0.573).Conclusions The frequency of syncopal episodes in children with VVS displayed a clear circadian rhythm,with a peak in the morning,but the POTS patients did not show a circadian variation.The male children with NMS tended to have a higher proportion of episodes in the morning than the female patients.