Pericardial cysts are usually detected by chance are are clinically silent in most cases. Nevertheless, symptoms and serious complications may occur. We describe a case of pericardial cyst diagnosed in an 8-year-old boy who was admitted with chest pain. Echocardiography revealed a mild to moderate pericardial effusion and a 7.5 x 5.5 cm intrapericardial echo-free lesion consistent with a pericardial cyst. Surgery was carried out 3 days afterward because of the patient's worsening condition, the progressive increase of pericardial effusion, and the onset of initial signs of cardiac tamponade. The cyst showed a long and easily movable vascular pedicle and inflammatory areas involving the pericardial surface. Like the pericardial effusion, the contents of the mass appeared as serosanguineous fluid on aspiration. Histologic examination confirmed the diagnosis of pericardial cyst and showed findings according to ischemia-related lesions of the cyst. The coexistence of pericardial cyst and cardiac tamponade is very unusual. The atypical anatomy and clinical course suggest a distinct and so far undescribed pathogenetic mechanism for this association: the torsion of a vascular pedicle and the subsequent development of ischemia-related lesions of the cyst.