Objectives. We sought to identify age-related differences in the ventricular response of patients after bidirectional cavopulmonary anastomosis (CPA) and to compare changes in the ventricular response among children <3 years of age who underwent CPA with that of age-matched control subjects who had a systemic to pulmonary artery shunt alone. Background. Pre-Fontan CPA has been advocated over a systemic to pulmonary artery shunt alone in patients with a single ventricle to facilitate ventricular volume unloading and minimize risk of the Fontan operation. Methods. Our study evaluated 23 patients who initially received a systemic to pulmonary artery shunt as an initial procedure before subsequent Fontan palliation. In eight of these patients (group I), bidirectional CPA was performed before age 3 years, and in four (group II), it was performed after age 10 years. The remaining II patients (group III, age and weight control group for group I) were maintained with their initial shunt until they underwent Fontan palliation. Serial echocardiographic analysis was used retrospectively to evaluate left ventricular volume and mass and systolic pump function (ejection fraction) before and after bidirectional CPA. Results. Through 10 months of follow-up, group I patients showed significant decreases in indexed end-diastolic volume both after CPA (120 ml/m1.5 body surface area vs.78 ml/m1.5, p = 0.001) and in comparison with values in patients in groups II and III, who showed no changes in end-diastolic volume (p < 0.001). Indexed ventricular mass decreased moderately after bidirectional CPA in group I (from 228 g/m1.5 body surface area to 148 g/m1.5) but remained unchanged in groups II and III. The differences in trends between groups I and III were significant (p = 0.03). Ejection fraction decreased significantly in group II versus group I patients (0.48 to 0.27 vs. 0.51 to 0.52, p < 0.05) after CPA. Oxygen saturation measurements before and after bidirectional CPA revealed a significant increase in group I (73% to 86%, p < 0.001) and a decrease in group II (82% to 73%, p < 0.01). Conclusions. Bidirectional CPA facilitates ventricular volume unloading and promotes regression of left ventricular mass in younger children (<3 years) in preparation for a Fontan operation. In contrast, bidirectional CPA is of questionable value in older children as a staging procedure for Fontan palliation.