Atrial tachyarrhythmias (ATs) contribute substantially to morbidity in adult patients with secundum atrial septal defects (ASDs). The purpose of this study was to prospectively determine the incidence of AT in adults with an ASD and identify predictors of AT occurrence after closure. This was a prospective study of 200 adult patients undergoing closure of a secundum ASD. Arrhythmic events were defined as sustained or symptomatic AT requiring treatment. Twenty percent of patients (mean age 50 +/- 17 years; 26% men) referred for ASD closure had a history of AT. Early follow-up was available for 90% of patients, and the prevalence of AT was 17%. Of 171 patients with late follow-up (mean 1.9 +/- 0.9 years), data were available for 90%. AT was detected in 16% of these patients. Closure resulted in alleviation of symptoms (p <0.001), but symptoms alone did not identify patients at risk of recurrent AT. After closure of the ASD, the likelihood of remaining arrhythmia free was highest in patients without a history of AT (p = 0.001) and those <40 years at closure (p = 0.04). In conclusion, transcatheter ASD closure in patients without a history of arrhythmias and those <40 years of age conferred the highest likelihood of a patient remaining arrhythmia free in follow-up. An arrhythmia-specific treatment strategy should be considered for patients with documented established AT before ASD closure, in addition to shunt relief.