<p>Cor triatriatum sinistrum is an uncommon congenital heart disease. The surgical repair outcomes are often satisfactory without reintervention. We report a case of residual cor triatriatum sinistrum after atrial septal defect repair in an adult. He was hospitalized to the emergency because of persistent palpitation. In classic cor triatriatum, all pulmonary veins return to proximal chamber (accessory left atrium), but this case we reported with left upper pulmonary vein returned to distal chamber (true left atrium). The transthoracic echocardiography and reconstructed computed tomography image illustrates the previous atrial septal defect patch and residual cor triatriatum sinistrum with obstruction, causing arrhythmia and symptoms. Thus, the patient underwent reoperation for cor triatriatum sinistrum repair. It is reasonable to believe that the fibromuscular membrane could be mistaken as the base of the left atrium during the first operation because the fenestration is so close to the atrial septum. Owing to the unique and deceptive anatomy, our report suggests that the inlet and outlet of the left atrium must be adequately detected during any atrial septal defect repair operation. A computed tomography scan is necessary when cor triatriatum sinistrum is suspected during the preoperative evaluation of patients with atrial septal defects.</p>