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 middle age patients with syncope during 24-hour electrocardiography (ECG) monitoring. Methods: 176 patients with syncope were evaluated tilt table test and 24-hour ECG monitoring. Middle age (from 30 year-old to 59 year-old) patients were enrolled and divided into two groups according to the results of tilt table test. The 24-hour ECG monitoring was performed in both groups, and R-R intervals were analyzed by time domain and frequency domain methods. Results: Tilt table test positive group (n=90, mean age: 45.9짹8.3) and tilt table test negative group (n=86, mean age: 46.9짹8.1) had no significant difference in time domain parameter. The high frequency band of frequency domain methods is showed significant difference with two groups(positive group 11.6짹4.6ms vs. negative group 13.1짹5.6ms, p=0.047), however there is no significant difference in low frequency, very low frequency components and low frequency/high frequency ratio. Conclusion: The HRV analysis during 24-hour ECG monitoring with the tilt-positive middle age patients was showed significantly decreased parasympathetic activity and these findings were manifested by frequent-domain parameters of HRV.