The interatrial septum (IAS) was studied by subxiphoid two-dimensional echocardiography (S2DE) in 88 infants under 12 months of age who weighed 1.2-9.1 kg. The IAS was adequately displayed in 87 of 88 patients. The morphology, presence and localization of defects in the IAS were evaluated by S2DE and retrospectively related to the findings at cardiac catheterization. In seven patients with no interatrial communication at cardiac catheterization, the IAS was straight, with an area of central thinning corresponding to the veil-like cover of the septum primum over the foramen ovale. The morphology of the IAS with a stretched, patent foramen ovale (56 patients) indicated the coexistence of a right or left ventricular volume or pressure overload, and was readily distinguishable from the IAS with a secundum type communication (13 patients). In patients with a stretched, patent foramen ovale and left ventricular overload lesions, the IAS was a nearly homogenous, curvilinear structure bowing into the right atrium, with a small area of septal dropout at the superior rim of the septum primum. In the presence of right ventricular overload lesions, the central defect of the foramen ovale was associated with a redundant flap valve of the septum primum billowing into the left atrium. In secundum type communications, the centrally located defect represented a deficiency rather than a redundancy of the septum primum. Balloon atrial septostomy (BAS) in 17 patients produced a secundum-type defect bordered by the flail remnants of the torn septum primum. Blalock-Hanlon septectomy (two patients) resulted in a large, posterior, sinus venosus-type communication which incorporated the preexisting BAS. Ostium primum defects (seven patients) were distinguished from the secundum lesions by their eccentric position in the IAS adjacent to the atrioventricular ring.