Abstract Background As previously known, patients with prolonged atrial fibrillation often have atrial thrombus. However, in children, cardiogenic embolism due to atrial fibrillation is rare. Also, the diagnosis of residual thrombus in the left atrial appendage is sometimes difficult even by transesophageal echocardiography (TEE). We have obtained a meaningful comparison between TEE findings and surgical specimen of bilateral atrial appendage. Case report A 12-year-old boy admitted to the nearby hospital, because of sudden onset of right-side hemiplegia. Till then, he was born at normal weight and grew up without any problem. The electrocardiogram indicated atrial fibrillation. He was transferred to the pediatric cardiology department of a hospital with higher function for the treatment of cardiogenic thromboembolism. After 72 hours from the onset of first thromboembolism, a new embolic event happened in his left brachial artery. The contrast-enhanced computed tomography (CT) showed thrombus of bilateral atrium and coronary sinus. Consequently, residual thrombus led to the potential risk of additional embolization. Therefore, catheter cerebral thrombectomy was performed, and then surgical thrombectomy for the bilateral atrium was planned. Preoperative CT showed thrombus in the left atrial appendage (LAA). However, intraoperative TEE showed no obvious thrombus in the LAA. Intricate pectinate muscle formation in the left atrial appendage almost disappeared (figure1A), which suspected remaining thrombus. In addition, massive thrombus was seen in the right atrium and coronary sinus. Coronary sinus thrombus was removed as far as possible, and bilateral atrial appendage resection and myocardial biopsy were performed. Simultaneously, modified-Maze was also done. Heart rhythm returned into sinus rhythm after surgery. Pathological findings did not show specific findings of myocarditis or pericarditis. Until now, the cause of thrombotic factors has not been fully clarified. Although the time of onset is unknown, probably, heart failure was triggered by atrial fibrillation, and consequently thromboembolism occurred. In the resected left atrial appendage, small thrombus remained among pectinate muscle (figure1B); therefore, postoperative anticoagulation should be needed. Conclusion We experienced a boy with thrombus in bilateral atrial appendages. In the resected left atrial appendage, multiple small thrombi remained in the pectinate muscle. From our experience, the disappearance of comb-like form in pectinate muscle by TEE suspects the residual thrombus, especially in the case of cardiogenic embolism. Abstract P1472 Figure.