Abstract
Congenital pericardial cysts are rare anomalies caused by the failure of fetal lacunae to coalesce into pericardial coelom. In this article a 9-year-old boy admitted with complain of palpitation in daily activities. The electrocardiography detected sinus tachycardia of 150 beats per minute with normal axis. Although chest X ray were normal, echocardiography showed an abnormal mass that compressed the posterior wall of left ventricle. The mass was extrinsic and confined to the pericardium. After midsternotomy, a huge cyst was found and totally excised. The complications of pericardial cyst can be significant, and the diagnosis relies on a careful examination and radiographic findings.