Ambulatory blood pressure monitoring (ABPM) was performed in 564 healthy schoolchildren during normal circadian activities. The data of two cohorts (155 boys and 139 girls aged 9-13 years and 184 boys and 168 girls with a body height between 120 and 155 cm) are presented. From the age of 9 to 13 years the mean 24-h systolic/diastolic blood pressure (SBP/DBP) increases from 107 +/- 9/66 +/- 7 mmHg to 115 +/- 13/68 +/- 9 mmHg in boys and from 104 +/- 5/64 +/- 6 mmHg to 109 +/- 8/65 +/- 9 mmHg in girls. When related to body height the values rise from 105 +/- 6/64 +/- 6 mmHg at 120 cm to 113 +/- 8/67 +/- 7 mmHg at 155 cm in boys and from 100 +/- 7/65 +/- 7 mmHg to 112 +/- 9/66 +/- 9 mmHg in girls. In comparison with the causal blood pressure data obtained from European studies, the presented ABPM values (daytime BP) are higher throughout, which may be explained by the increased activity during daytime with ABPM. There is a mean difference of 4.4 mmHg in boys and of 3.0 mmHg in girls for SBP and of 10.8 mmHg in boys and of 9.0 mmHg in girls for DBP when related to age. In relation to body height, there is a mean difference of 4.4 mmHg in boys and of 3.5 mmHg in girls for SBP and of 10.9 mmHg in boys and of 10.5 mmHg in girls for DBP. We conclude that standards derived from causal blood pressure measurements should not be used for the evaluation of ABPM data.