The aim of this study was to assess blood pressure variability (BPV) and its determinants in untreated hypertensive children. The study group consisted of 124 children, 91 boys and 33 girls, aged 14.9+/-2.5 with essential hypertension and no use of antihypertensive drugs. The subjects underwent routine examination, blood tests and ambulatory blood pressure (BP) monitoring. BPV was defined as the value of the standard deviation of BP for day- and night time periods. Daytime BPV was higher than night time BPV, and systolic BPV was higher than diastolic BPV. Significant positive correlations between 24-h, day- and night time systolic blood pressure (SBP), but not for diastolic blood pressure (DBP), and BPV were observed. In univariate analysis, day- and night time systolic BPVs were correlated with fasting glucose (r=0.609, P=0.02 and r=0.439, P=0.04); daytime systolic BPV, daytime diastolic BPV and night time systolic BPV were correlated with birth length (r=0.428, P=0.04; r=0.426, P=0.04 and r=0.439, P=0.04, respectively), and night time systolic BPV and night time diastolic BPV were correlated with age (r=0.604, P=0.02 and r=0.833, P=0.0001). However, in multiple linear regression analysis, daytime diastolic BPV was determined only by gender and systolic 24-h BP; night time systolic BPV depended on age, daytime SBP and DBP values, and daytime SBP and DBPs were determinants of night time diastolic BPV. The results highlighted the complex nature of BPV, with favourable role of host factors in its aetiology. The determinants of BPV in children are consistent with those in adults. Relationships between BPV and its determinants in untreated hypertensive subjects ought to be investigated in further researches.