Blood pressures (BPs) were taken with a mercury sphygmomanometer and an automatic recorder on 3524 children representative of an entire geographic community. For all children ages 5, 8, 11, and 14 years in the initial examination, age-specific systolic and diastolic (4th phase) selected percentiles were assessed. Of these children, 1101 were reexamined after one year. Observations from a group of 35 fifth-graders examined monthly for eight months were pooled to observe intra-child BP variability. This estimate was used to reduce to zero in a statistical adjustment the regression toward the mean of the BPs for the after-one-year reexamined children. It was found that those children initially in the top ten percentiles had, upon reexamination, on the average only 3 mmHg lower systolic and 1 mmHg lower diastolic levels. In a multiple regression analysis, the previous year's BP contributed a partial correlation coefficient of 0.6--0.7 for each age cohort to the variability of the BP, controlling for other determinants. These findings, based on reliable, basal-like measurements, point to a degree of persistence which is quite high. The higher the degree of tracking the more likely that primary hypertension begins early in life.