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How the amplitude of the R wave in lead V_5(RV_5)of the ECG represents the left ventricular(LV)mass was investigated in 894 students aged 15(boys : 545, girls : 349). The influence of body composition[ie, percentage of body fat(%fat)]was taken into consideration. A significant correlation was found between RV_5 amplitude and LV mass for both genders. However, the relationship was stronger for boys than for girls. The students were then divided into 3 groups ; that is, those at either the top or bottom 10th percentile(low or high-fat range)and the rest of the students(middle-fat range), depending on the % fat calculated by the bioelectrical impedance method. The significant correlation between RV_5 and LV mass was found only for the 2 groups of boys whose percentage fat was in the low or middle-fat range. The correlation coefficients were 0.40 and 0.34, respectively. Moreover, in the boys' low-fat range, the RV_5 of students whose LV mass was large(390th percentile), was significantly higher(p<0.01)than in the other ranges. Setting a particular cutoff point of RV_5 in the low-fat range of boys improved sensitivity as well as specificity(20-30% better among all boys)for detecting large LV mass. These observations suggest that classification of subjects by body composition could be improve the reliability of ECG assessment for left ventricular hypertrophy, although the gender and number of subjects in whom improvement is expected are limited.