To investigate the effect of obesity and hypertension on left ventricular (LV) mass in children, we performed echocardiography and measured the height, weight, and blood pressure of 267 healthy children (145 boys and 122 girls) aged 12 years. The percentage of body fat was estimated using bioelectric impedance to derive the total adipose weight and lean body weight. End-diastolic measurements of LV parameters were obtained from M-mode echocardiograms. The LV mass was calculated by using the formula of Devereux et al. A strong positive correlation was demonstrated between non-normalized LV mass and height or other measures of body size. Systolic blood pressure was weakly correlated with non-normalized LV mass in boys. The impact of height on LV mass differed between boys and girls. Thus, different allometric formulas to normalize the LV mass for height were determined, using the height to the 3.1 and 1.9 powers for boys and girls, respectively. Regression analysis revealed that only total adipose weight affected the normalized LV mass, and that the effect of total adipose weight was greater in girls than in boys. The obese children had a significantly greater normalized LV mass than the nonobese children. The increase in the LV mass due to obesity appeared to be eccentric, because of the lack of an association between the indices of obesity and relative wall thickness. Our data indicate that appropriate normalization of LV mass is necessary for each study population, and that LV hypertrophy due to obesity begins in childhood.