In mass screening examination of heart diseases,395 apparently healthy children of school age were given echocardiographic examination. Out of the data obtained,left ventricular muscle volume (LVMV) was calculated in 301 cases (169 males and 132 females). LVMV was in good correlation with body surface area, and sexual and racial differences were apparent to some extent. These data provide knowledge of normal range of LVMV in Japanese children of school age.