We reviewed our experience using the Amplatzer septal occluder (AGA Medical, Golden Valley, MN, USA) to close large, secundum-type atrial septal defects (ASDs) in children. Between June 2002 and December 2005, 52 patients (mean age 13.5 +/- 8.7 years) underwent transcatheter closure of large (> or =25 mm), secundum ASDs with the use of the Amplatzer septal occluder (ASO). Groups 1 and 2 included patients with a retroaortic rim of <5 mm (n = 39) or > or =5 mm (n = 13), respectively. All procedures were performed with general anesthesia and transesophageal echocardiographic guidance except for 10 patients, which involved local anesthesia and three-dimensional transthoracic echocardiography. Successful device implantations, device sizes, approaches, complications, and closure rates were assessed. Device implantation was successful in 50 patients (96.1%), with no difference between groups (95% vs 100%, P>0.05). In 2 patients, implantation failed because of embolism or deployment failure. Device were larger in group 1 than in group 2 (29.7 +/- 4.2 vs 26.7 +/- 3.8 mm, P = 0.04). The right upper pulmonary-vein approach was more common in group 1 than in group 2 (P = 0.0001). Complications and closure rates did not differ between the groups (P > 0.05). Transcatheter closure of large, secundum ASD by using the ASO device was feasible, and complication rates were low. A deficient retroaortic rim did not preclude successful device implantation; however, a large device may be needed to close large ASD. Close long-term follow-up is necessary to determine the safety of transcatheter closure of large ASDs in children.