It is critical to distinguish between anatomic [obstructive anterior tricuspid valve leaflet (ATL) or pulmonary atresia (PA)] and functional (tricuspid insufficiency with elevated pulmonary vascular resistance) right ventricular outflow tract (RVOT) obstruction in cyanotic infants with Ebstein's anomaly (EBS). We reviewed 4 cases of EBS to determine the accuracy of 2-D echo in distinguishing anatomic from functional RVOT obstruction. Two of 4 were newborns. The other 2 were children (8; 12 y.o.) with severe cyanosis as neonates, requiring Waterston shunt. EBS with PA was the 2-D echo diagnosis in the 8 y.o., and an obstructive ATL in the 12 y.o. Both were confirmed at operation. The 2 newborns had 2-D echo findings suggesting obstructive ATL with minimal pulmonary valve (PV) motion. Both had shunt operation. One died, and, at autopsy, the ATL was found to be non-obstructive. The other had a 2-D echo 1 month post-operatively, revealing only mild pulmonary valve thickening.In cyanotic neonates with severe forms of EBS, functional RVOT obstruction can be mistaken for anatomic by 2-D echo. Apparently obstructive ATL and minimal PV motion could be the result of reduced antegrade blood flow. Prior to operation, presumed anatomic obstruction should be confirmed by cardiac catheterization and angiocardiography.