The Monodisk, a device to facilitate percutaneous closure of cardiac septal defects, was developed and tested in vitro and in vivo. Atrial septal defects (8-10 mm) were created in five dogs using transcatheter techniques. The defects were then closed with the Monodisk. The device was easily and successfully fixed in place in all dogs. The efficacy of the device was determined radiographically. No shunting of contrast medium was observed in any case. Animals were followed for 6 months. Results showed good biocompatibility and several advantages of the device: it requires only a simple one-step placement procedure; it utilizes a small delivery system; it is easy to place and detach; it is self-centering and stable; and it can be repositioned or retrieved prior to detachment. Its possible clinical applications include correction of atrial septal defects, ventricular septal defects, patent ductus arteriosus, as well as creating an aortopulmonary window.