Sinus venosus defects are relatively uncommon forms of interatrial communication in which one of the caval veins has a biatrial connection, and partial anomalous pulmonary venous connections are the rule. The variety related to the inferior caval vein is much the rarer of the two forms, and here the defect is posterior and inferiorly placed. To determine whether an inferior sinus venosus defect could be identified preoperatively, we reviewed retrospectively the available echocardiograms and cineangiograms of the five patients with an inferior sinus venosus defect who underwent surgical repair between January 1983 and December 1991. A cross-sectional echocardiogram had been performed preoperatively in four of these patients, but the correct diagnosis had been made in only one case. Retrospective study, however, showed that the extent of the true atrial septum and the nature of the junction of the inferior caval vein with the atriums could be evaluated from the subcostal position in three of these four patients. Each of the three had an interatrial communication located posteriorly and inferiorly adjacent to the atrial connection of the inferior caval vein, along with abnormally connected right pulmonary veins. The oval fossa was intact and the coronary sinus was normal in all. Cardiac catheterization had been performed in three of the five patients. Anomalous return of right pulmonary veins was noted in each, and the posterior and inferior location of the interatrial communication was evident in two. An interatrial communication adjacent to the atrial connection of the inferior caval vein, along with anomalous connection of the right-sided pulmonary veins, are the diagnostic features of the inferior variety of sinus venosus defects. These features can be demonstrated by cross-sectional echocardiography from the subcostal position.