This study reports the results of a Japanese clinical trial of transcatheter closure of atrial septal defects (ASD) using the Amplatzer septal occluder (ASO). Thirty-five patients with secundum ASD underwent transcatheter closure using the ASO at a median age of 12.9 years (range, 3.2-29.2 years) and a median weight of 39.2kg (range, 11.6-65.1 kg). The ASO was successfully implanted in 34 patients. The mean ASD diameter of the 34 patients measured by transesophageal echocardiography was 11.7 +/- 4.2mm (range, 5.0-20.8mm) and the mean balloon stretched diameter was 16.8 +/- 4.2 mm (range, 9-25 mm). The mean ASO size was 16.9 +/- 4.3 mm (range, 9-26mm). Complete closure rate at 1 day and 1 year after closure was 91% and 97%, respectively. One patient developed a transient second-degree atrioventricular block during the implantation procedure. No other complications occurred. Transcatheter closure of ASD using the ASO is effective and safe.