A 2-year-old boy in whom idiopathic enlargement of the right atrium was diagnosed in utero by fetal echocardiography underwent surgical intervention because of progression of right atrial dilatation. During operation, the lateral right atrial wall was externally reinforced after partial resection by approximating and fixing the neighboring autologous pericardium around the external circumference of the right atrium. This technique is a useful means of preventing recurrence of dilatation in histologically abnormal right atrial tissue that predisposes the patient to possible reintervention.