Development of aortopulmonary collaterals during the course of surgical palliation for single-ventricular anatomy has been linked to adverse outcomes following Fontan palliation. We investigated the hemodynamic significance of aortopulmonary collaterals during presurgical cardiac catheterization of patients with single-ventricle surgically palliated anatomy. Thermal indicator dilution studies were performed to determine degree of shunt. A total of 52 patients were studied and the data were analyzed. Measurements by thermal indicator dilution correlated significantly with qualitative angiographic grading of aortopulmonary collaterals. However, the hemodynamic significance of these aortopulmonary collaterals, as measured by thermal indicator dilution, did not correlate with postoperative outcome variables. This study demonstrated that thermal indicator dilution measurement holds promise for hemodynamically quantifying the significance of aortopulmonary shunts in surgically palliated single-ventricular patients. However, the degree of recirculation from aortopulmonary collaterals does not correlate with outcome after Fontan surgery in this patient cohort.