Left atrial appendage aneurysm is a rarely reported condition. Symptoms are absent in childhood and diagnosis is usually incidental. Systemic embolization or arrhythmia can bring these cases to medical attention. We report the case of a 12-year-old male with massive left atrial appendage aneurysm who presented with effort intolerance and supraventricular arrhythmia. The diagnosis was made by transthoracic echocardiography. Magnetic resonance imaging and left atriogram were also done before surgical resection.