The aim of this study was to assess the heart rate variability in patients with vasovagal syndrome (VVS). Heart rate variability was expressed as: (1) the standard deviation (SD) of the mean RR interval; and (2) the SD as a percentage of the mean RR interval (%SD). Heart rate variability was measured in VVS patients and compared with control individuals. Eighteen patients (mean age 50 +/- 14 years) with a history of recurrent syncope and positive tilt testing were included in the study. Fifteen asymptomatic individuals (mean age 53 +/- 13 years) with no history of syncope and negative tilt testing were used as a control group. The SD and %SD (39 +/- 38 and 5 +/- 4 msec) in the VVS group were statistically higher at the tenth minute of tilt testing than in the control group (20 +/- 14 and 2.5 +/- 1.8 msec, P = 0.03 and P < 0.05, respectively). The mean RR interval (mean heart rate) was shorter after the 15th minute of tilt testing in the VVS group than in the control group (RR-VVS 687 +/- 136 msec, RR-control 801 +/- 131 msec, P < 0.05). It is concluded that heart rate variability, as expressed by the SD of the mean RR interval, and the SD as a percentage of the mean RR interval (%SD) are significantly higher in VVS patients than in control asymptomatic individuals.