There is a growing body of evidence that interventricular dyssynchrony results in abnormal ventricular mechanics that may be detrimental to myocardial performance.(1; 2) Adult patients with LBBB, dilated cardiomyopathy and CHF are increasingly being treated with a novel pacing technique in order to restore electrical and mechanical synchrony.(3-5) This technique, biventricular or resynchronization pacing, involves the insertion of a specialized cardiac pacing system that not only maintains atrioventricular synchrony, but also restores interventricular synchrony by simultaneously activating the right and left ventricles. There have been a number of clinical trials (MIRACLE, MUSTT, INSYNC) that have revealed marked clinical improvement in adult patients with moderate-to-severe CHF.(6-8) There have been a few small reports of the acute effects of biventricular pacing in pediatric patients with acquired interventricular dysynchrony either from repair or palliation of congenital heart defects, or from chronic standard pacing from one ventricular chamber (single-site ventricular pacing).(9-11) One of the main hurdles encountered when evaluating these patients is the lack of reliable, reproducible non-invasive imaging modalities. Recent reports have shown promising results with a relatively new technique known as tissue Doppler imaging (TDI).(12-14) TDI echocardiography has recently been employed as a non-geometric measure of myocardial systolic and diastolic performance for both the left and the right ventricles. Utilizing low velocity signals from discrete points on the myocardial walls, motion can be assessed under a variety of conditions non-invasively. Additionally when two points along a ventricular wall are simultaneously assayed, the relative motion between the two points can estimate myocardial systolic strain in the direction of the Doppler sound beam. This technique eliminates any variation due to the translational motion of the heart.