Aims: To assess the validity of QTc dispersion to cardiac autonomic nervous function status. Method and results: Prospective comparisons of QTc dispersion measurements in 77 neurally mediated cardiac syncope (NMCS) patients (29 boys, 48 girls, mean age 13.4 +/- 0.78) were evaluated. Head-up tilt test (HUT) was positive in 38 patients and negative in 39 patients. QTc dispersion was significantly higher early morning and late night in HUT-positive group compared with HUT-negative group (p < 0.001). Although there was a circadian rhythm in HUT-positive group, there was no change on the QTc dispersion within a day in the HUT-negative group. QTc dispersion in the HUT-positive group was high in accordance with their high sympathetic nervous stimulation. Moreover, QTc dispersion was higher during late night and early morning than the rest of the day. Conclusion: These results may explain why the HUT-positive patients usually have their syncope attacks early in the morning. Additionaly, we can use QTc dispersion measurement as a new noninvasive electrocardiographic test to evaluate cardiac sympathetic nervous system in NMCS. (c) 2004 Elsevier Ireland Ltd. All rights reserved.