AIM: Data on arrhythmia outcome following device closure of atrial septal defect (ASD) are lacking. This study provides medium-term follow-up data on atrial arrhythmias in patients who were ≥40 years of age at the time of transcatheter ASD closure. METHODS AND RESULTS: It is a retrospective review. Mean age of the 159 patients was 57 years. Median follow-up was 3.6 years (range 6 months - 10.9 years). Patients were classified, according to arrhythmia status prior to ASD closure, into Group I, no history of atrial arrhythmia (n = 119, mean age 55.5 years); Group II, paroxysmal atrial arrhythmia (n = 18, mean age 55.7 years); and Group III, persistent atrial fibrillation (n = 22, mean age 65.7 years). Group III patients were significantly older, had larger left atrial size, and had higher mean pulmonary arterial pressure than Group I and II patients (P <0.001). Prior to closure, radiofrequency ablation was carried out in 12/18 (66%) of Group II and 3/22 (14%) of Group III. After device closure, 7 patients (6%) of Group I developed new atrial fibrillation. Fifty per cent (9/18) of Group II but only 9% (2/22) of Group III were in sinus rhythm on follow-up. CONCLUSION: Device closure alone in patients with persistent atrial arrhythmia is not likely to restore sinus rhythm in the medium term. New atrial arrhythmia occurred in 6% of patients who were in sinus rhythm prior to device closure. At least 50% of the patients with paroxysmal atrial arrhythmia continue to have significant atrial arrhythmia following device closure, and the role of ablation prior to closure in patients with a history of arrhythmia requires refinement.