An improved 10 Fr version of the atrial septal defect (ASD) occlusion system consisting of two umbrellas for transvenous introduction over the long veno-arterial guide-wire was used to attempt closure in five adult patients with large defects (26-35 mm). The umbrellas are made of nitinol wire frame and a thin membrane of microporous polyurethane. Supported by the metal cannula and guided by selective left atriography, the umbrellas of 45-60 mm were placed individually into the atria and screwed together at the septum level by means of a torquer catheter. Positioning and screwing on, unscrewing, separating, and repositioning the umbrellas up to 17 times were needed to anchor the prosthesis correctly in a patient. The prosthesis could be implanted primarily in all patients (in one at second session). Dislodgement of a 60 mm prosthesis and left atrial perforation with a 55 mm prosthesis required surgery in two patients 8 hours and 2 weeks post procedure, respectively. A single umbrella-arm fracture was noticed in one patient 4 months after the implantation. All five patients were free of symptoms at follow-up after 7-10 months. Transcatheter closure of large ASDs is technically feasible with this system. The morbidity is mainly associated with the implantation of very large umbrellas.