Background: Heart rate variability (HRV) has become a commonly used but difficult-to-interpret means of studying the interplay between the autonomic nervous system and cardiovascular function. The purpose of this study was to evaluate the heart rate variability in patients with syncope during tilt-table testing. Methods: This study included 30 patients (mean age 39.2짹17.3 years) with positive tilt testing and 26 individuals (mean age 38.9짹16.4 years) with negative tilt testing. In tilt-positive patients, HRV analysis was performed during 5 minutes in the supine position and just before syncope. In tilt-negative patients, HRV analysis was performed during 5 minutes in the supine position and 15 minutes with tilting and administration of nitrate. Time and frequency domain parameters were evaluated. Results: Baseline results were similar in both groups. Within 5 minutes preceding syncope or after 15 minutes with tilting and nitrate, root mean square successive difference (RMSSD; 21.5짹12.3 vs. 43.0짹41.0, p = 0.016) and number of adjacent normal RR intervals >50 ms (NN50; 11.2짹12.2 vs. 39.2짹43.8, p= 0.015) were significantly shorter in tilt-positive patients as compared to tilt-negative patients. Other parameters in time domain and frequency domain showed no difference between two groups. Conclusion: The HRV analysis during tilt-testing with the tilt-positive middle age patients showed significantly decreased parasympathetic activity just before syncope and these findings were manifested by time-domain parameters of HRV.