OBJECTIVES: This phase I study attempted to report the initial safety and efficacy results of transcatheter closure of perimembranous ventricular septal defects (PmVSDs) using the new Amplatzer Membranous VSD Occluder (AGA Medical Corp., Golden Valley, Minnesota) in the U.S. BACKGROUND: The most common congenital heart disease is PmVSD. Surgical repair is widely accepted, but still carries a small but definite risk of morbidity and mortality. METHODS: Between October 2003 and August 2004, a total of 35 patients with PmVSD underwent an attempt of transcatheter closure under transesophageal and/or intracardiac echocardiographic guidance. The median age was 7.7 years (range, 1.2 to 54.4 years) and median weight was 25 kg (range, 8.3 to 110 kg). The median Qp/Qs ratio was 1.8 (range, 1 to 4), and the median VSD size as assessed by echocardiography was 7 mm (range, 4 to 15 mm). RESULTS: The attempt to place a device was successful in 32 patients (91%). The median device size used was 10 mm (range, 6 to 16 mm). The complete closure rates by echocardiography at 10 min (transesophageal/intracardiac), 24 h, 1 month, and 6 months (transthoracic) were 47% (15/32), 63% (20/32), 78% (25/32), and 96% (27/28), respectively. The median fluoroscopy time was 36 min (range, 14 to 191 min), and the median total procedure time was 121 min (range, 67 to 276 min). Three patients (8.6%) had serious adverse events of complete heart block, peri-hepatic bleeding, and rupture of tricuspid valve chordae tendineae. No other patient encountered serious adverse events during the follow-up. CONCLUSIONS: Transcatheter closure of a PmVSD is technically feasible and seems safe enough in children over 8 kg in weight to warrant continuation of clinical trials to assess the long-term safety and efficacy.