A 28-year-old man visited the emergency department of our hospital with a chief complaint of palpitation and chest pain. The patient had undergone 4 operations at other hospitals for tetralogy of Fallot (TOF), left pulmonary atresia, an aberrant right coronary artery, and a right aortic arch. As a result of thorough investigations, we suspected that the cause of the patient's symptoms was an excess of the right ventricular pressure over the left ventricular pressure, which was caused by right ventricular compression resulted from an abnormal mass on the anterior surface of the right ventricle, and by pulmonary stenosis (PS) associated with right ventricular outflow tract stenosis (RVOTS). Excision of the mass, right ventricular outflow restoration (RVOTR), and pulmonary valve replacement (PVR) were indicated. The mass on the anterior surface of the right ventricular was found to have been caused by retention of serous fluid in the interstice formed by a folded expanded polytetrafluoroethylene (ePTFE) pericardial sheet. An ePTFE pericardial sheet, which is used to supplement the pericardium, has been reported to have advantages with respect to prevention of adhesion, denaturation of pericardial substitutes, and inflammatory thickening and adhesion of the epicardium, compared with other materials used as pericardial substitutes. However, epicardial thickening has been noted with the use of ePTFE pericardial sheets, and hence, its use is currently avoided in many cases. This case presents an extremely rare pathology in which the inflammatory reaction of the epicardium caused by an ePTFE pericardial sheet is suspected to have caused serous components to become tightly encapsulated in the interstice formed by the folded patch ; no definite cause was identified. Thus far, no other such case has been reported, and ePTFE pericardial sheets should be used with caution.