Objectives The objective of this study was to understand the expected hemodynamic responses to exercise in children and adolescents with myocardial dysfunction. Methods and Results With the use of Doppler and M-mode echocardiography, cardiovascular changes during maximal semisupine exercise in 11 patients (7 to 17 years old) with myocardial dysfunction were compared with those of a healthy control group (n = 11). Endurance fitness and mean values for maximal cardiac index, stroke index, heart rate, peak aortic velocity, and left ventricular shortening fraction were all significantly lower in the patients (P < .05). Stroke volume rose at the onset of exercise in both groups; whereas values were subsequently stable in the control subjects, stroke volume declined at high-intensity exercise in the patients. Conclusions These findings imply that augmented myocardial contractility is necessary to sustain stroke volume during exercise. Moreover, the results suggest that pattern of stroke volume response to exercise may serve as a useful marker of myocardial function in children with heart disease. (Am Heart J 1999;137:126-33.)