We investigated the influence of body composition on electrocardiographic (ECG) detection of left ventricular (LV) hypertrophy in 894 high-school students. The percent body fat and LV mass were estimated by bioelectrical impedance and echocardiography, respectively. There was no significant difference in LV mass among subjects classified according to the percent body fat. The cutoff ECG amplitudes (RV5 and SV1+RV5) for detection of LV hypertrophy (LV mass >/=90th percentile in both boys and girls) with 90% specificity were highest in the low-fat group and lowest in the high-fat group. When the effects of the percent body fat on ECG amplitudes were taken into account, the sensitivity of the ECG for detection of LV hypertrophy improved from 32.7 to 38.2% in boys; however, no improvement was observed in girls (from 33.3 to 30.6%). Evaluation of the percent body fat may improve the efficacy of ECG detection of LV hypertrophy in adolescent boys, but the usefulness of this method may be limited in girls.