The aims of this study were to determine the prevalence and predisposing conditions for atrial fibrillation (AF) in adults with atrial septal defect (ASD) and to evaluate the influence of age at surgical repair. The study population consisted of 286 adults with ASD (mean age 39.5 +/- 19 years). All patients had >or = 1 follow-up visit and a Doppler echocardiographic study. One hundred ninety-two of the patients underwent surgical closure 1 to 34 years before the study. Analyzed variables were entered into univariate (Mann-Whitney U) and multivariate (stepwise logistic regression) models to assess independent predictors for AF. The prevalence of AF was similar in surgically treated patients (15.6%) and in the nonsurgical group (13.8%) (p = 0.69). Multivariate analysis showed that current age (RR 1.9 per each decade of age, 95% confidence interval [CI] 1.3 to 2.7, p = 0.001), mitral regurgitation (RR 3.0 per each degree of regurgitation, 95% CI 1.6 to 5.8, p = 0.001), left atrial enlargement (RR 2.8 per each 10 mm increase in size, 95% CI 1.5 to 5.2, p = 0.001), and tricuspid regurgitation (RR 1.9 per each degree of regurgitation, 95% CI 1.0 to 3.7, p = 0.04) were independent predictors of AF; however, gender, anatomic type, defect size, Qp:Qs, pulmonary artery pressure, right ventricular dimension, left ventricular shortening fraction, and prior surgical repair were not related to late AF development. In the surgical group, age >25 years at the time of surgery was the only predictor for AF independent of age at the time of the study (p = 0.02).