A 10-year-old girl presented with a 2-month history of atrial fibrillation. Chest x-ray film showed marked cardiomegaly (A). Transthoracic echocardiography showed a giant (13 × 10 cm) saccular aneurysm, related to and communicating with the left atrium (LA) through a 3.5-cm neck with dense spontaneous echo contrast and a large (5.5 × 7.3 cm) thrombus (B, Online Videos 1, 2, and 3). Left ventricular (LV) function was markedly impaired, with anterior wall akinesia. Coronary angiography revealed displacement and compression of the left anterior descending (LAD) coronary artery due to a mass effect from the aneurysm (C, arrows). Diagnosis of a giant LA appendage aneurysm was made intraoperatively (D). The aneurysm was resected and the aneurismal neck (E, arrows; asterisk indicates thrombus) was closed with an autologous pericardial patch (F). The post-operative course was uneventful, and the patient regained sinus rhythm, with improvement of global and regional LV function.