A prospective M-mode echocardiographic study was done to look for early cardiovascular changes in children prone to hypertension with blood pressures between the 75th and 95th percentiles for age. Average systolic/diastolic pressures in 27 children with borderline hypertension were 137/89 mm Hg compared to 110/68 mm Hg for the 26 controls. Echocardiographic measurements were normalized for comparison using two methods. The borderline hypertensive group mean values were significantly greater than controls for left ventricular wall thickness (p less than 0.05 for method 1; p less than 0.001 for method 2), left ventricular mass (p less than 0.001; p less than 0.005) and left ventricular wall thickness to radius ratio (p less than 0.001, both methods). Echocardiographic estimates of left ventricular function were lower in the hypertensive group. This study suggests that cardiac hypertrophy can be shown by noninvasive means in some children before arterial pressure becomes elevated. To assess the incidence and possible consequences of early target organ changes, more extensive clinical evaluation of borderline hypertension in children is recommended.