The decision to close an ostium secundum atrial septal defect by interventional catheterisation implies knowing its size, form and the relationship of its borders to neighbouring structures as accurately as possible. Three-dimensional echocardiography provides unique views of the interatrial septum and the authors set out to assess its performance. Ten patients, aged 8 to 20 years, included in a multicenter European clinical trial of closure of atrial septal defects with the CardioSEAL prostheses, were examined by transoesophageal echocardiography with three-dimensional reconstruction of the interatrial septum viewed from the left or right atrium. The septal defect had a very variable morphology, round, oval raquet-shaped and occasionally multiple. The surface area of these defects varied by about 70% during the cardiac cycle, maximal during ventricular systole and minimal during atrial systole. The maximal diameter measured by two-dimensional transoesophageal echocardiography underestimated that measured by three-dimensional echocardiography by about 30%. Two patients had a juxta-aortic caudal border or a juxta-superior vena caval cephalic border making the defect unsuitable for catheter insertion of a CardioSEAL occluder. On the other hand, another patient had an adequate juxta-aortic border although it seemed too narrow with conventional imaging techniques. The authors conclude that three-dimensional reconstruction of transoesophageal echocardiography is the best method of selecting candidates for closure of ostium septum atrial septal defect by intervantional catheterisation.