Giant intraatrial septal aneurysm originating from a branch of the left coronary artery

We present the case of a 38-year-old woman who had a large intraatrial aneurysm occupied by old thrombosis. The aneurysm was successfully removed, and the atrium was repaired. Pathohistological findings indicated that the inflow artery of the aneurysm had an anomalous origin from the left main coronary artery, and its pathogenesis was unknown. It is occasionally difficult to distinguish a large coronary aneurysm from a mediastinal tumor because this aneurysm is a rare entity, even more so in an atrial septum. A giant coronary aneurysm should be considered an alternative diagnosis in the event of a mediastinal mass. Surgery is recommended for a large coronary aneurysm.