Transcatheter closure of a secundum atrial septal defect was attempted using a prototype clamshell septal umbrella closure system in 11 patients (six males, five females, aged 4.4-15.0 years and weighing 16-51 kg). The balloon stretched atrial septal defects were 10.3 to 20 mm in diameter, and 23 to 40 mm diameter umbrellas were used. All patients improved clinically after the procedure. Fixed splitting of the second heart sound disappeared after closure in 10 patients, systolic murmur disappeared in nine, diastolic rumble in nine, and paradoxical ventricular septal motion in seven. Doppler color flow imaging immediately after implantation showed significant residual shunt in all patients; however, no shunt flow was observed in seven patients and minimal residual shunt was present in four one year after the procedure. Nine of 11 umbrellas demonstrated fracture of the stainless steel arm confirmed by chest radiography and cinefluorography. Arm fracture occurred between 1 week and 12 months after implantation. None of the devices were dislodged and no other complications were noted despite the fractures. Transcatheter closure of atrial septal defect is a feasible and useful procedure but the present device needs further modification.