A 15-month-old boy with atrial fibrillation was found to have gross cardiomegaly on a chest radiography; further evaluation by echocardiography showed a giant right atrial aneurysm which is in the form of a cyst like lesion. The patient underwent successful surgical reduction of the right atrium converting his heart rhythm to normal. The operative measure was taken to prevent thrombus formation in the right atrium and lower the risk of atrial arrhythmias. The morphologic features of the resected atrial tissue showed a thin wall with a central aneurysm and focal endocardial fibrosis without inflammatory response consistent with a diagnosis of idiopathic dilatation of the right atrium.