We investigated the changes of left ventricular isovolumic relaxation time (LVIRT) with growth and the factors influencing them during childhood, in a 10-year follow-up study. We measured the body height (BH), body weight (BW), lean body weight (LBW), heart rate (HR), blood pressure (BP), left ventricular muscle volume index (LVMVI), and LVIRT corrected by the preceding R-R interval (IRT/R-R), of 187 healthy children at 3-year intervals (6, 9, 12, 15 years old). IRT/R-R were prolonged with growth in boys and girls. At the ages of 12 and 15, the IRT/R-R of girls were longer than those of boys. Significant correlation coefficients for IRT/R-R on systolic and diastolic, BW, and LVMVI were 0.28, 0.31, 0.20, 0.28, respectively. These data suggest that (1) IRT/R-R is mainly determined by the diastolic BP, (2) left ventricular diastolic function of children develops with growth of body size, especially BW, and (3) left ventricular early diastolic function in adolescents is related to sexual maturation.