Congenital aneurysm of the left atrial appendage can be caused by congenital dysplasia of the pectinate muscles and may be accompanied by a congenital absence of the pericardium. Symptoms generally manifest after two decades and the most common symptom is atrial arrhythmia in the form of atrial fibrillation / flutter. A four year-old patient with no symptoms underwent an echocardiographic examination for the investigation of a heart murmur. Echocardiographic examination revealed a large cystic lesion occupying the left hemithorax and compressing the left ventricle. The patient was referred to our center. The lesion caused displacement of the heart rightward behind the sternum which made the examination difficult. We aimed to present this rare case of giant left atrial appendage aneurysm in the light of current literature.