Dysplasia of the leaflets is a well-known integral part of Ebstein's malformation. It is less well-recognized that occasionally the septal leaflet may be completely absent and not simply displaced from its usual annular attachment. This may make it difficult to differentiate by echocardiography between Ebstein's malformation and the rare lesion in which the tricuspid orifice is completely devoid of leaflet tissue ("congenitally unguarded orifice"). A neonatal case is described in which a partially unguarded orifice was present in association with pulmonary atresia and intact ventricular septum but misdiagnosed as Ebstein's malformation. In addition we reviewed the morphologic features of all 46 cases of pulmonary atresia and intact septum in the Cardiopathological Collection of the Children's Hospital of Pittsburgh: 17 also had Ebstein's malformation, while three had a congenitally unguarded tricuspid orifice. We found that the differential diagnosis occurred when the right ventricle was dilated rather than being a hypoplastic cavity, as was a feature of all the cases with unguarded orifice and five of those with Ebstein's malformation. The difference between the lesions is best demonstrated by examining the mural leaflet of the valve, which is absent when the orifice is unguarded but displaced in association with Ebstein's malformation. This feature should be recognizable by cross-sectional echocardiography. We have confirmed previous studies that dilatation of the chambers of the right heart indicates a very poor prognosis in cases with pulmonary atresia and an intact ventricular septum.