Study aim: To examine the differences between the second and first measurements of blood pressure and to show the necessity of designing a standardised protocol.Material and methods: Anthropometric data of 1618 children were assessed. Blood pressure and heart rate values were determined twice after 5-min sitting with a sphygmomanometer (Omron M5-I, Germany). The differences between the second and first measurements were assessed using Student's t-test.Results: A group of 921 boys and 697 were studied. Mean systolic blood pressures in the first and second measurements were 124.9 ± 15.7 and 121.4 ± 14.9 mm Hg, respectively, mean diastolic blood pressures - 78.0 ± 13.6 and 76.0 ± 12.4 mm Hg, respectively, the between-measurement differences being highly significant (p<0.001). No significant differences were noted for the heart rate. The prevalence of hypertension assessed from the first and second measurement was 23.9 and 4.4%, respectively.Conclusion: The results point to the indispensability of designing a valid, standardised protocol of conducting blood pressure measurements in paediatric epidemiological studies.