A 2-year old asymptomatic girl is presented in whom the chest x-ray film led to the discovery of a grossly distorted cardiac shadow. Angiocardiography showed a giant aneurysm originating from the left atrium. At operation an intrapericardial aneurysm of the left atrial appendage, measuring 12 X 4 X 4 cm, was found. It communicated with the left atrium through an orifice measuring 3 cm. The aneurysm was excised.