A heart murmur was detected in a 4-day-old newborn after birth. Consequently, a comprehensive cardiology examination with transthoracic echocardiography was performed. The initial examination raised suspicion of a tumour-like mass in the left hemithorax and also detected severe mitral insufficiency. However, a subsequent CT imaging ruled out the presence of a left hemithorax tumour and instead confirmed the presence of a congenital aneurysm of the left atrial appendage (a). The aneurysm, measuring 35 × 26 mm, caused compression of the left ventricular cavity (b). However, the lack of electrocardiography-synchronised imaging precluded assessment for potential thrombi. Nevertheless, the patient underwent successful surgical resection of the left atrial appendage, with a favourable postoperative outcome. Left atrial appendage is a relatively rare condition that can lead to serious complications such as tachyarrhythmias and thromboembolism. It is often misdiagnosed as tumour expansion. Left atrial appendage is frequently an incidental finding on CT angiography, which plays a crucial role in its diagnostic process. However, electrocardiography synchronisation is not necessary during CT angiography for left atrial appendage diagnosis.