Aortic stenosis is a progressive disorder and can be the cause of serious arrhythmias and possibly sudden death. Evaluation and follow-up of the autonomic nervous system may provide some useful information for management of the disease. Our study aimed to examine heart rate variability in children with aortic stenosis in the supine position and to detect the changes in autonomic activity during head-up tilt testing. Sixteen patients and 11 healthy controls participated in the study. In the supine position, seven minutes of continuous echocardiographic (ECG) recording was performed, followed by four consecutive ECG recordings, each consisting of seven minutes in 70 degrees tilt position. To obtain power spectrums, the tachograms were taken on the autoregressive mode. The mean RR interval duration, standard deviation of RR interval, central frequencies of low and high frequency oscillations, their powers, total power and percents of normalized low and high frequency powers were accepted for statistics. There were no significant differences between the groups in the supine position. In tilt position, mean RR interval and its standard deviation were decreased in both groups. The central frequency of low freuquency power significantly (p<0.05) shifted to left, normalized low frequency power increased and normalized high frequency power decreased in the control group at the beginning of tilt position, but at the second phase of tilt position in the patient group. We conclude from the results that children with mild-to-moderate aortic stenosis reflect delayed response to sympathetic provocation.