Background: There is a lack of evidence regarding treatment options for adults with an atrial septal defect (ASD) who present with an open defect or with sequelae after closure of the defect. The aim of this study was to describe the clinical characteristics and treatment of a large cohort of adult patients born with an ASD type II. Methods and results: Data on the clinical characteristics of 882 ASD II patients (mean follow-up of 4.2 years) included in the Euro Heart Survey on adult congenital heart disease were analysed. At baseline, the defects of 377 patients (mean age 39.2 (16.1) years; 65% females) had been closed, leaving 505 patients (mean age 41.1 (16.4) years; 68% females) with an open ASD. Hemodynamic abnormalities were more prevalent among patients with an open compared to those with a closed defect at baseline: pulmonary arterial hypertension 35% versus 13%; right ventricular (RV) dysfunction 31% versus 8%; and severe RV volume overload 18% versus 1% (all P-values<0.001). These prevalences increased with age, but hemodynamic parameters remained stable during follow-up in nearly all patients with a small defect. Also functional limitations were more common in those with open defects at baseline compared to those with closed defects (54% versus 25%). There was no difference in the prevalence of arrhythmia's. The best independent predictors of functional limitations appeared to be PAH (odds ratio 25.2 (5.8-109.6); P<0.001)) and RV volume overload (odds ratio 2.3 (1.5-3.4; P<0.001)) in a multivariable model. During follow-up, 9 patients died and in 294 patients the defect was closed, in 180 patients surgically, and in 114 patients by device. Among the latter group there were relatively more females (78% vs 66%; P=0.035). In the surgically closed group defects were more "severe". There were substantial differences according to country in the relative frequency of device closure versus surgical closure, as well as the size and hemodynamic severity of the defects closed. Conclusion: The data from this study provide a cross-section of the kind of adult patients with an ASD that are seen at outpatient clinics for adult congenital heart disease throughout Europe. Taken together, non-operated patients fared significantly worse in all aspects of hemodynamics studied than the patients whose defects had been closed. In moderate or large defects, when not-operated, clinical parameters tend to worsen with time, and closure of such a defect-the sooner the better-seems always to be the preferred treatment option. In the majority of small defects, operation is not necessarily indicated. (C) 2007 Elsevier Ireland Ltd. All rights reserved.