Blood pressure (BP) data obtained during a BP screening program were analyzed to determine the prevalence of "significant" hypertension, defined by the Second National Heart, Lung, and Blood Institute Task Force on Blood Pressure Control in Children-1987 as the level of BP above which medical evaluation and intervention are recommended. In 14,686 black and white St. Paul and Minneapolis schoolchildren aged 10 to 15 years, BP was measured twice during an initial screening examination. Children with systolic BP in the upper 30 percentiles of distribution after the initial screening had their BP remeasured two additional times at a rescreening examination. The prevalence of significant hypertension was determined according to BP criteria established by the Task Force report. After the two screening BP measurements were averaged, significant systolic hypertension was found in 1.0%, significant diastolic hypertension in 3.5%, and significant systolic or diastolic hypertension, or both, in 4.2% of the students. After the rescreening examination, the percentage of students remaining with significant hypertension was reduced to 0.3% for systolic, 0.8% for diastolic, and 1.1% for systolic or diastolic hypertension, or both. These results show that significant hypertension is uncommon in pre-high-school students and confirm the need for repeated BP measurements to make an accurate diagnosis of hypertension. However, the results should not detract from current recommendations to monitor BP in children on a yearly basis to detect longitudinal BP tracking patterns that may be consistent with early essential hypertension.