Idiopathic right atrial enlargement is a rare cardiac anomaly presenting as a giant right atrium. A 20-year-old woman presenting for pneumothorax was found to have a pericardial mass compressing right ventricle. Echocardiography and magnetic resonance suggested the diagnosis of lone right atrial appendance enlargement. The mass was surgically removed and histopathology confirmed the diagnosis of right atrial appendance aneurysm. The case is notable for its rarity.