Atrial fibrillation was found in 19% of a series of 32 adults with large atrial septal defects. This confirms the previously held impression that atrial fibrillation is not uncommon in older patients with this lesion. Atrial fibrillation, in the presence of a large atrial septal defect, correlates well with left atrial pressure elevation and left atrial enlargement. The presence of atrial fibrillation, in the face of an isolated, large atrial septal defect, may be considered evidence for left ventricular failure. Advancing age appears to be an additional factor correlating with the presence of atrial fibrillation. Other factors may occasionally precipitate atrial arrhythmias.