Elevated systemic blood pressure is one of the most important risk factor of diabetic nephropathy. The aim of the study was to estimate the influence of systemic blood pressure on renal function in children and adolescents with type 1 diabetes mellitus. Fifty-nine patients without evidence of arterial hypertension were recruited. In all patients 24-hour automatic blood pressure monitoring and renal examination (GFR, ERPF, FF, renoscintigraphy, urinary albumin excretion) were performed. The patients were divided into three groups according to blood pressure load: group I (less than 40% of systolic blood pressure--SBP and diastolic blood pressure--DBP values above 90th percentile for sex, age, height and body weight)--26 persons, group II (more than 40% DBP above 90th percentile)--25 persons, group III (more than. 40% SBP and DBP above 90th percentile)--8 persons. The study suggests that 24-hour automatic blood pressure monitoring is useful for early detection of increased blood pressure in diabetic children and adolescents. The patients with elevated both systolic and diastolic blood pressures had more frequently glomerular hyperfiltration. The persons with elevated only diastolic blood pressure had the lowest glomerular filtration and filtration fraction.