INTRODUCTION: Uncontrolled arterial hypertension brings direct and long-term sequelae in adult age, such as stroke, ischaemic heart disease with myocardial infarction, left ventricular hypertrophy or cardiac arrhythmia. AIM: To assess heart rate variability (HRV) spectral parameters and left ventricular mass in slim children with arterial hypertension, and to search for correlations between these two parameters. METHODS: 35 children aged 14.4+/-3.1 with idiopathic untreated arterial hypertension were enrolled. The control group included 30 age- and gender-matched healthy children (aged 14.1+/-2.9 years). In all analysed subjects an analysis of HRV parameters (high frequency (HF) and low frequency (LF) components) during 10-minute waking state and sleeping time was performed and left ventricular mass (LVM) as well as the left ventricular mass index (LVMI, g/m) were assessed based on echocardiographic measurements. RESULTS: There was no difference in LF during the waking state and sleep HF between the two groups, whereas HF values during the waking state were significantly lower (p<0.05) in children with hypertension. The LF/HF index from both registration intervals was significantly higher in the group of children with hypertension. In children with hypertension, LVM and LVMI correlated significantly with LF (r=0.32, p<0.05 and r=0.39, p<0.01). LVM and LVMI correlated positively with the LF/HF index during night hours (r=0.45, p<0.004 and r=0.49, p<0.002). No significant correlations were found between the analysed parameters in children from the control group. CONCLUSIONS: The increase of sympathetic activity during sleep correlates significantly with left ventricular mass and corrected left ventricular mass index in children with arterial hypertension.