In patients eventually requiring Fontan operation, preoperative ventricular dilation is a risk factor for poor postoperative outcome. Bidirectional cavopulmonary anastomosis (BCPA) has been advocated to reduce ventricular volume load and decrease ventricular dilation; however, this effect has not been documented. To determine the effectiveness of this operation at reducing ventricular size while maintaining ventricular function, we reviewed preoperative and early postoperative echocardiograms in 20 patients undergoing BCPA. Ten of these patients also had later follow-up studies. Ventricular size was assessed by measurement of end-diastolic and end-systolic area, and heart rate was recorded at each study time. Ventricular function was assessed by area shortening. Indexed end-diastolic area was significantly smaller after BCPA than before (26.2 +/- 10.5 vs 21.4 +/- 6.5 cm2/m2). Area shortening and heart rate did not change significantly. Patients who had prior systemic-to-pulmonary shunts had a significant decrease in end-diastolic area after BCPA (30.2 +/- 12.2 vs 22.4 +/- 7.3 cm2/m2); those with prior pulmonary artery banding did not have a significant decrease (22.5 +/- 7.2 vs 19.9 +/- 6.5 cm2/m2). Patients with ventricles having left ventricular morphologic features had a significant decrease in end-diastolic area after BCPA (29.2 +/- 10.8 vs 22.8 +/- 6.5 cm2/m2); those with right ventricular morphologic features did not have a significant decrease (19.2 +/- 5.9 vs 18.0 +/- 5.9 cm2/m2). We conclude that BCPA results in a significantly reduced systemic ventricular preload and size while normal ventricular performance is maintained. Other factors such as prior palliation and ventricular morphologic features may affect the magnitude of the decrease in ventricular size seen after BCPA.