Objective: The myocardial performance index (MPI) measures the ratio of isovolumic time intervals to ventricular ejection time. The effects of altered ventricular preload or afterload on MPI have yet to be determined. This study was designed to determine the impact of altered preload on left and right ventricular myocardial performance index in the clinical setting of left-to-right lesions. Materials and Methods: The left and right ventricular myocardial performance indexes were measured in 17 patients with atrial septal defect (ages 6 to 148 months), 23 patients with ventricular septal defect (ages 2 to 160 months), and 24 healthy children (ages 3 to 160 months). A complete 2- dimensional and Doppler echocardiographic examination was performed in all study groups. Results: In patients with atrial septal defect, ventricular septal defect, and control group subjects, the left ventricular MPI was 0.38, 0.37 and 0.32, respectively, and the right ventricular MPI was 0.24, 0.21, and 0.20, respectively. No significant differences in the left and right ventricular myocardial performance indexes were seen between patients with left-to-right shunt lesions and control subjects. Conclusion: This study documents that the myocardial performance index is a quantitative measure of ventricular function that appears to be relatively independent of changes in preload.