A direct effect of obesity on myocardial function has not been not well established. Our aim was to investigate the effect of body mass index (BMI) and homeostatic model assessment of insulin resistance (HOMA-IR) on left-ventricular (LV) myocardial function in normotensive overweight and obese children by tissue Doppler imaging (TDI). We calculated the mitral annular displacement index (DI) and myocardial performance index (MPI) using TDI indices of systolic and diastolic LV function. In this hospital-based, prospective cross-sectional study, we studied 60 obese (mean age 13.2 +/- A 2.0 years) and 50 normal children. Subjects were divided into three groups: group 1 (BMI < 25, n = 50, control), group 2 (BMI 25-29.9 kg/m(2), n = 30, overweight), and group 3 (BMI a parts per thousand yen 30 kg/m(2), n = 30, morbidly obese). Standard echocardiography showed increased LV diameters and LV mass/index and preserved ejection fraction in obese children. By TDI, LV systolic and diastolic function showed that peak late myocardial velocity (Em = 15.4 +/- A 2 cm/s), peak early myocardial velocity (Am = 8.7 +/- A 1.3 cm/s), Em/Am ratio (1.8 +/- A 0.3), isovolumetric relaxation time (IVRT = 59.2 +/- A 8.2 ms), MPI (0.39 +/- A 0.03), and DI (25.5 +/- A 3.2 %) of the lateral mitral annulus in the obese subgroups were significantly different from those of control subjects (18.2 +/- A 1.2 cm/sn, 6.9 +/- A 0.6 cm/sn, 2.6 +/- A 0.2, 51.2 +/- A 9.6 ms, 0.34 +/- A 0.03, and 33.13 +/- A 5.0 %, respectively; p < 0.001). These structural and functional abnormalities were significantly related to BMI. There were positive correlations between HOMA-IR, septal MPI, and LV mass. DI and MPI data indicated impaired subclinical LV function in all grades of isolated obesity at a preclinical stage. Insulin resistance and BMI correlated significantly with indices of LV function.