To establish the diagnostic criteria for a sinus venosus atrial septal defect cross sectional echocardiograms, cineangiograms, and surgical notes of all patients with this diagnosis seen at the Children's Hospital of Pittsburgh between 1986 and 1988 were reviewed. Seven patients were identified. In each the extent of the atrial septum and the nature of the junction of the superior vena cava with the atria were evaluated echocardiographically from the subcostal position. All had overriding of the superior vena cava and abnormally connected right pulmonary veins. Six patients had undergone cardiac catheterisation and cineangiography. Five patients underwent surgical repair. The operative findings were consistent with the expected morphology in all five, and these features were additionally confirmed in a specimen from the cardiopathological museum. Therefore, the basic anatomical feature of a superior sinus venosus interatrial communication is a biatrial connection of the superior vena cava. This, together with anomalous drainage of the right sided pulmonary veins, results in an interatrial communication outside the confines of the true atrial septum. Overriding of the superior vena cava across the upper rim of the oval fossa is suggested as the pathognomonic diagnostic feature that can clearly be demonstrated echocardiographically from the subcostal position. In essence the lesion is an interatrial communication rather than an atrial septal defect.