OBJECTIVE: The aim of this study was to investigate the heart rate variability (HRV) parameters in patients with neurally mediated reflex syncope. METHODS: Thirty-three patients (10 men, age range 16-50 years) who were scheduled to undergo head-upright tilt test (HUTT) with a typical history of vasovagal syncope (VVS) underwent 24-hour Holter monitoring in an attempt to study HRV parameters. Sixteen individuals without syncope and similar baseline characteristics made up the control group. RESULTS: Seventeen of the 33 syncopal patients showed HUTT positivity. All the time domain HRV parameters except the mean RR interval were higher in the HUTT (+) syncopal patients than in the negative responders. Although not statistically significant, the positive responders had higher NN50, pNN50, RMSSD and SDNNi values than the control group. HUTT (-) VVS patients had lower SDNN and SDANN values when compared with the control group. The other HRV parameters did not differ significantly between the HUTT (-) patients and the control group. CONCLUSION: Increased parasympathetic tonus as reflected by significantly higher HRV parameters is associated with a greater frequency of HUTT positivity in patients with syncope and no organic heart disease.