Transcatheter closure of secundum atrial septal defect (ASD) using clamshell or buttoned devices is accompanied by a high incidence of residual shunt. Recently, a new self-centering device, the Amplatzer septal occluder (ASO), has been evaluated in an animal model with very good results. Therefore, our purpose is to report on our initial clinical experience with this device. Thirty patients underwent an attempt at catheter closure of their ASDs at a median age of 6.1 yr (range, 2.9-62.4 yr) and median weight of 22 kg (range, 13-69 kg) using the ASO. The median ASD diameter measured by transesophageal echocardiography (TEE) was 12.5 mm (range, 5-21 mm), and the median ASD balloon stretched diameter was 14 mm (range, 7-19 mm). All patients had right atrial and ventricular volume overload with a mean +/- SD Qp/Qs of 2.3 +/- 0.6. A 7F catheter was used for delivery of the device in all patients. The device was placed correctly in all patients. There was immediate and complete closure (C) in 17/30 patients, 10 patients had trivial residual shunt (TS), and 3 had moderate residual shunt (MS). The median fluoroscopy time was 15 min (range, 8-35 min), and the median total procedure time was 92.5 min (range, 40-135 min). There was no episode of device embolization or any other complication. Follow-up was performed using transthoracic echocardiography (TTE) 1 day, 1 mo, 3 mo, and yearly thereafter. At 1 day, there was C of the ASD in 24/30 patients, 3 had TS, 1 had small shunt (SS), and 2 had MS. At a median follow-up interval of 6 mo, there have been no episodes of endocarditis, thromboembolism, or wire fracture. We conclude that the use of the new ASO is safe and effective in complete closure of secundum ASDs up to a diameter of 21 mm in the majority of patients. Further clinical trials are underway.