The aim of this study was to evaluate the HRV at rest and during tilt test (HUTT) in children with a history of vasovagal syncope and to link the HRV indices with the clinical results of the test. HRV indices were assessed in the supine position and during the initial 5 minutes of the 60-degree HUTT in 49 patients (33 females, 16 males, mean age of 13 +/- 2.8 years) who were evaluated for recurrent syncope, The positive to negative results of the test were 21 to 28. The normalized power of high frequency component (npHF) decreased, normalized power of low frequency component (npLF) and the LF:HF ratio increased during HUTT of tilt-positive patients (P < 0.05 for each parameter). Parallel changes, but to a lesser degree, were observed for similar HRV parameters of tilt-negative patients, In addition, the HF and all the time-domain indices decreased significantly (P 0.05) during HUTT in the latter group. When the tilt-positive and -negative patients were compared, the npHF was lower (P = 0.002), npLF and LF:HF ratio were higher (P = 0.01 and P = 0.001, respectively) during the test in tilt-positive patients, reflecting increased sympathetic tone in this group. A cut-off point for LF:HF was assigned as 2.7 for differentiating tilt-negative and tilt-positive results. The specificity, sensitivity, and positive and negative predictive values of this cut-off point were calculated as 93%, 52%, 85%, and 41%, respectively. Patients with vasovagal syncope show variations in vagal autonomic tone and appear to be more prone to syncope when their sympathetic tone is elevated at the beginning of the test. LF:HF > 2.7 is a specific marker (specificity 93%) and can correctly predict a positive tilt test in 85% of patients.