Congenital aneurysms arising from the left sinus of Valsalva are extremely uncommon, and when unruptured often asymptomatic. Abnormalities of rhythm are more commonly associated with dilatation of the right coronary sinus although the reason for this is unclear. We report the first case of an unruptured aneurysm involving the left sinus of Valsalva presenting with paroxysmal atrial fibrillation. Two-dimensional echocardiography confirmed the diagnosis.