The purpose of the study was to evaluate the dynamics of diastolic and systolic function from rest to maximal exercise using conventional echocardiography and tissue Doppler imaging (TDI) in obese prepubertal boys compared to age-matched lean controls. Eighteen obese (10 with first degree obesity and 8 with second degree obesity according to French curves, BMI: 23.3+/-1.8 and 29.0+/-2.0 kg/m2, respectively) and 17 lean controls (BMI=17.6+/-0.6 kg/m2, P<0.001), aged 10-12 years were recruited. After resting echocardiography, all children performed a maximal exercise test. Regional diastolic and systolic myocardial velocities were acquired at rest and each workload. Stroke volume and cardiac output were calculated. At rest, obese boys had greater left ventricular (LV) diameters and LV mass. Boys in the first degree group showed no diastolic or systolic dysfunction, whereas boys with second degree obesity showed subtle diastolic dysfunction. During exercise, both obese groups showed greater stroke volume and cardiac output. First degree obese boys exhibited greater systolic and diastolic tissue Doppler velocities than controls, whereas second degree obese boys had lower diastolic tissue velocities irrespective of exercise intensity and lower fractional shortening at high exercise intensities than controls. In conclusion, no impairment in diastolic or systolic function is noticed in prepubertal boys with first degree of obesity. Enhanced regional myocardial function response to exercise was also demonstrated in this population, suggesting adaptive compensatory cardiac changes in mild obesity. However, when obesity becomes more severe, impaired global and regional cardiac function at rest and during exercise can be observed.