BACKGROUND AND OBJECTIVES: Syncope is defined as a sudden temporary loss of consciousness associated with a loss of postural tone with spontaneous recovery. It is a common clinical problem with complex and heterogeneous etiologies, but vasovagal syncope is the main cause of unexplained syncope. Bradycardia and hypotension by transient dysfunction of cardiac autonomic nervous system have been cited as the main pathophysiology of the vasovagal syncope. Therefore, we studied whether analysis of heart rate variability (HRV) by 24-hour ambulatory ECG monitoring would reflect autonomic imbalance between cardiac sympathetic and vagal efferent activity in the patients of vasovagal syncope. MATERIALS AND METHOD: 45 patients (male=2, female=3, mean age=2.214 years) with syncope were enrolled, and divided into 2 subgroups according to the results of head-up tilt test: head-up tilt test positive (group S1) and negative (group S0). A sex-matched control group consisted of 9 healthy volunteers (male=, female=, mean age=16 years, Group C). The 24-hour ambulatory ECG monitoring was performed in all groups, and R-R intervals were analyzed by time- and frequency-domain methods. The time-domain measurements of HRV were mean NN(mean of all coupling intervals between normal beat), ASDNN(mean of the standard deviations of all normal R-R intervals for 5-minute segments of the entire recording), SDNN(standard deviation of all normal R-R intervals over 24 hours), SDANN(standard deviation of average R-R intervals in all 5 minutes segments of the entire recording), rMSSD(square root of the mean squared differences of successive R-R interval) and pNN50(percent of differences between adjacent normal R-R intervals more than 50ms during 24 hours), and frequency-domain measurements were low frequency (LF), high frequency (HF) components and LF/HF ratio. RESULTS: The LF/HF ratio was significantly higher in syncope patients with positive results of head-up tilt test and syncope patients with negative results than in control (p<0.05). The LF, HF, mean NN, ASDNN, SDNN, SDANN, rMSSD, and pNN50 were not significantly different among these three groups. CONCLUSION: These results suggest that the cardiac autonomic nervous system in patients with vasovagal syncope has sympathetic-activated balanced without changes of total power of both sympathetic and parasympathetic components.