The purpose of this study was to assess the prevalence of left ventricular hypertrophy in adolescents with persistently elevated blood pressures. Chest roentgenograms, electrocardiograms, and echocardiograms were performed on 50 adolescents with elevated blood pressure and 50 matched normotensive control subjects. No subject in either group demonstrated cardiomegaly on x-ray. Interpretation of the electrocardiograms indicated that similar numbers of both hypertensive (7/50) and control subjects (8/50) had ECG evidence of left ventricular hypertrophy. The echocardiograms showed that the mean left ventricular wall thickness (LVWT) in the hypertensive adolescents was 7.8 mm ± 0.1 (SE), compared with 6.5 ± 0.1 in the control subjects (P < .001). When the measurements were indexed to body surface area, the difference remained highly significant. Indexed left ventricular mass (LVM)/body surface area (BSA) was also significantly greater (P < .001) in the hypertensive (84.2 gm/sq m ± 2.1) than in the control subjects (72.0 ± 2.1). Using data from the normotensive control subjects, we defined the 95th percentile for both LVWT/BSA and LVM/BSA. Among hypertensive adolescents, 9/50 had LVWT/BSA and 8/50 had LVM/BSA above this level. For control subjects, only 1/50 had elevated LVWT/BSA values and 2/50 elevated LVM/BSA values. This study demonstrates that hypertensive adolescents have an increased prevalence of left ventricular hypertrophy and that echocardiography is the most useful noninvasive method to detect these changes.