Despite a good haemodynamic result, many children have amildly decreased arterial oxygen saturation following a total cavopulmonary connection. Our study was performed to determine possible mechanisms of right-to-left shunting in these patients. We performed elective cardiac catheterization in 19 children at a mean interval of 3.6 years following a total cavopulmonary connection. The intrapulmonary right-to-left shunt, the intracardiac right-to-left shunt and the total right-to-left shunt were calculated under mechanical ventilation with 100% oxygen. The intrapulmonary right-to-left shunt was 10.8±3.5% of the pulmonary blood flow, and the total right-to left shunt accounted for 18.9±5.2% of the systemic blood flow. The intracardiac right-to-left shunt in patients with no relevant venovenous collaterals or leaks in the atrial tunnel was calculated at 6.4±3.0% of the systemic blood flow, while the intracardiac right-to-left shunt in patients with relevant collaterials or leaks accounted for 13.0±5.9% of the systemic blood flow. Since intrapulmonary arteriovenous fistulas were not demonstrated angiographically in any of our patients, the intrapulmonary right-to-left shunt is probably due to low ratios of perfusion to ventilation in some pulmonary segments. The intracafdiac right-to-left shunt was due to leaks across the interatrial baffle, collaterals between stystemic and pulmonary veins, and to the coronary sinus draining to the pulmonary venous atrium.