Adults with isolated secundum atrial septal defect (ASD) may present with paroxysmal atrial tachyarrhythmias. The clinical course of these rhythmic disturbances after correction of the ASD is unknown. Of 188 patients aged 44 years old or older with isolated ASD, 27 (14%) (mean age 52.7 years, range 44 to 71) had documented paroxysmal atrial tachyarrhythmias preoperatively: 16 had paroxysmal atrial fibrillation, 2 had atrial flutter, and 9 had supraventricular tachycardia. Among these 3 groups, there were no differences in age, sex, New York Heart Association class, duration of preoperative symptoms, mean right atrial and pulmonary arterial pressures, shunt size, atrial size, or follow-up period. After operative repair, follow-up data were available in all patients for a mean of 12 years (range 1.5 to 25). Of the 16 patients with preoperative paroxysmal atrial fibrillation, 14 (88%) continued to have increasingly frequent paroxysmal atrial fibrillation, culminating in sustained atrial fibrillation. In 1 of the 2 patients with paroxysmal atrial flutter, sustained atrial fibrillation developed. Of the 9 patients with preoperative paroxysmal supraventricular tachycardia, 5 had no further episodes of this arrhythmia postoperatively.