Neurocardiogenic syncope may be caused by enhanced sympathetic activity evoking a vasodepressor - cardioinhibitory reflex. Heart rate variability(HRV) methods can be used to assess the modulation of sympathetic and parasympathetic activity. To determine whether HRV measurements are related to the outcome of head-up tilt (HUT) we studied 29 syncopal patients aged 7-19y. After 30 min supine. patients were tilted to 80° for 30 min or until syncope occurred. Sequential beats free from ectopy were analyzed. Time domain indices included standard deviation (SDNN), root mean square successive differences (RMSSD), percent excceding 50msec (pNN50). An autoregressive model was used to calculate power spectra. Low frequency power (LFP,.05-.15Hz), high frequency power (HFP,.15-.40Hz), and total power (TP,.01-.40Hz) were compared. Data were obtained supine before tilt (baseline), within 5 min, 5-10 min, and 15 min after HUT or pre-syncope. Seventeen patients fainted (HUT+), and 12 patients did not (HUT-). Variability indices were markedly different for HUT- and HUT+ at baseline: SDNN was 123±17 vs 78±6ms, RMSSD was 127±23 vs 64±6 ms, pNN50 was 51±6 vs 31±4%, respectively. Spectral data demonstrated decreased HFP and TP in HUT+(834±133 and 2855±420 ms2) vs HUT- (3433±840 and 7062±1500). With tilt, SDNN, RMSSD and pNN50 decreased proportionately in HUT- and HUT+. However, sympathovagal balance, measured by the ratio LFP/HFP and by normalized LFP, was markedly increased in HUT+ (2.2±0.7 and 0.43±.03) compared to HUT- (0.8±0.2 and 0.31±.02) at baseline and differences increased with tilt. Data were not different early or late after HUT but with syncope, sympathetic activity decreased and parasympathetic activity increased. Decreased HRV with decreased parasympathetic activity and increased indices of sympathovagal balance prior to HUT predict a positive tilt test in children referred for evaluation of neurocardiogenic syncope.