In infants with a single right ventricle (RV), stage I palliation involves aortic reconstruction, systemic-to-pulmonary shunt placement, and atrial septectomy. Many require re-intervention for residual coarctation of the aorta (CoA). Doppler echocardiography can detect residual CoA in most infants, but its ability to predict severity has not been studied. This study compares gradients from Doppler interrogation to those from cardiac catheterization in infants with residual CoA. We performed a retrospective study of infants after stage I palliation from 2000 to 2014. Infants with an echocardiogram and catheterization before the second-stage palliative surgery were included. Infants with an echocardiogram >30 days before catheterization were excluded. Doppler-derived gradients were compared to catheterization-derived gradients. Echocardiographic assessment of tricuspid valve (TV) and RV function were recorded. The cohort included 95 infants, and thirty-three (35%) had CoA. Doppler-derived and catheterization-derived gradients correlated weakly in infants with CoA (r = 0.37, p = 0.036) and without CoA (r = 0.35, p = 0.005). Among infants with CoA, 17/33 had none or trivial tricuspid regurgitation (TR) and normal RV function, and Doppler-derived gradients correlated with catheterization gradients in this group (r = 0.71, p = 0.001). In 16/33 infants with ≥moderate TR or RV dysfunction, gradients did not correlate (r = −0.003, p = 0.992). After a stage I palliation in infants with single RV and CoA, Doppler-derived gradients poorly predicted the severity of CoA. Infants with normal TV or RV function had Doppler-derived gradients more predictive of catheterization-derived gradients. Doppler-derived gradients have limited utility in determining the severity of CoA after a stage I palliation.