Background: Systemic to pulmonary collateral flow (SPCF) is common in single ventricle patients. Because no validated method to quantify SPCF exists, neither its hemodynamic burden nor clinical impact can be systematically evaluated. We hypothesize that (1) the difference in total ascending aortic (Ao) and caval flow (SVC+IVC) and (2) the difference between pulmonary vein and artery flow (PV – PA) provide two independent estimators of SPCF. Methods and Results: We measured Ao, SVC, IVC, right (RPA) and left (LPA) PA, left (LPV), and right (RPV) PV flows in 23 single ventricle patients, 18 superior cavopulmonary (SCPC) and5 Fontan, during routine cardiac MRI studies using through-plane phase contrast velocity mapping. Two independent measures of SPCF were obtained: (1) Ao – (SVC + IVC). (2) (LPV-LPA) + (RPV-RPA). Values were normalized to BSA, Ao, and PV and comparisons made using regression and Bland-Altman analysis. SPCF ranged from 0.2-2.1 L/min for (1) and 0.2-2.2 L/min for (2) (average indexed SPCF of 0.4-5.8 L/min/m²). SPCF in SCPC patients averaged 38% of Ao and 55% of PV, compared to 24% (p=0.04) and 28% (p=0.003) respectively in Fontans, both statistically significant differences. There was strong linear correlation between the two methods of estimating SPCF (y=0.78x+0.24, r=0.79, p < 0.001). Conclusions: We present a novel noninvasive method for the SPCF quantification that should prove an important clinical tool. We show that SPCF is often a significant hemodynamic burden, especially in SCPC physiology. Future investigations will allow objective study of the impact of collateral flow on outcome.