The Tecumseh project investigates the evolution of hypertension in a healthy population. Of 946 subjects aged 18 through 38 years, 124 had clinic blood pressure readings higher than 140/90 mm Hg (the mean for borderline hypertensive subjects was 130/94 mm Hg). Compared with normotensive subjects, borderline hypertensive subjects had higher home blood pressures (mean, 12/7 mm Hg higher). Their childhood and postpubertal blood pressures were elevated (6/4 mm Hg higher than normal at age 6 years and 12/7 mm Hg higher than normal at age 21 years), and hypertensive target organ changes were detected. Borderline hypertensive subjects also had elevated minimal forearm resistance (0.22 U higher than normal), decreased stroke index (1.8 mL/m2 lower than normal), and impaired ventricular diastolic relaxation (mitral Doppler peak early diastolic blood flow [E] to peak late diastolic blood flow [A] ratio 0.13 lower than normal). Borderline hypertensive subjects had significant abnormalities in other coronary risk factors (cholesterol levels were 0.39 mmol/L higher, triglyceride levels were 0.45 mmol/L higher, high-density lipoprotein levels were 0.08 mmol/L lower, insulin levels were 38 pmol/L higher, and 16.5% more of them were overweight). Borderline hypertension is neither transient nor innocuous. Its association with other predictors of atherosclerosis calls for clinical attention.