Efforts to close left-to-right shunts at Ochsner Medical Institutions have been directed toward atrial septal defects (ASD) and patent ductus arteriosus (PDA). PDA's were constructed in dogs by interposing a segment of jugular vein between the aorta and main pulmonary artery. Five dogs in which the PDA was closed by a plug device inserted through the femoral vessels were put to death at 6 to 12 months. Histologic sections showed good fibrous ingrowth with endothelial covering on the aorta and pulmonary artery sides. There were no migrations redidual shunts. At cardiac catherterization, 18 patients had ASD's sized and located as to position in the septum. The sized ranged from 13 to greater than 30 mm. in diameter. The ASD sizes in patients who underwent standard operative closure were compared to the measurements at catheterization, and the variation was insignificant. In 5 patients, centrally positioned secundum ASD's were closed with double umbrella devices, 25 to 35 mm. in diameter. Anatomic contraindications for umbrella closure include ASD's greater than 30mm. in diameter, anomalous pulmonary venous connection, common atrium, inferiorly or superioly located secrumdum ASD, and sinus venosus ASD. Follow-up studies from 6 to 12 months on 5 patients with umbrella closure have revealed no hemolysis, arrhythmias, thromboembolism, migration, or other untoward effects.