Multiple longitudinal studies have demonstrated reasonable stability of blood pressure (BP) levels and hemodynamics throughout childhood and adolescence and into adulthood. Part of this stability might be caused by genetic factors that are expressed steadily over time. We aimed to determine the relative contributions of genetic and environmental factors to the stability of BP and underlying hemodynamic characteristics between ages 14 and 18 years. In addition, potential ethnic differences were examined. To this end, resting levels of BP and impedance-derived hemodynamic variables were measured twice in >500 pairs of European American (EA) and African American (AA) twins, with an intervening period of 4.1 years. Structural equation modeling of the twin data on BP and underlying hemodynamic variables (adjusted for age, sex, and body mass index) showed that heritabilities were moderate to high (25% to 64%) and relatively stable over time. These genetic influences accounted for 60% to 100% of the phenotypic tracking correlations (range 0.39 to 0.62). Emergence of novel genetic influences accounted for a significant part of the variance (17% to 33%) at the second measurement occasion. There were significant ethnic differences for BP, with nonshared environmental influences becoming larger over time in AAs compared with EAs for both systolic and diastolic BP. In summary, novel genetic effects emerge during development into adulthood and explain a considerable part of the variation in BP and hemodynamics. Environmental influences become more important with age in AAs compared with EAs for both systolic and diastolic BP. Future elucidation of these environmental factors may help explain ethnic differences in hypertension risk.