The first successful removal of an epicardial cyst is reported. Evidence is presented to demonstrate that the cyst developed as a result of an abnormality occurring within the first five weeks of fetal life.Pericardial cysts are well recognized, comprising 7 % of all mediastinal tumours (Nelson, Shefts, and Bowers, 1957). Unilocular, thin-walled sacs, they lie usually in the anterior costophrenic angle in close association with the pericardium. They contain clear fluid and occasionally communicate with the pericardial cavity.In contrast, a similar cyst arising inside the pericardial cavity, and attached to the heart, is exceedingly rare. Only three reports showing any resemblance to the case described below have been found in the literature. Gould (1960) described an intrapericardial cyst in a stillborn child, as well as a small multilocular cyst arising from the ventricle of an adult also found at necropsy. Piwnica and Menage (1962) described the successful removal of a large cyst arising from between the aorta and the pulmonary artery in a child. Preoperative irradiation had destroyed the cyst lining, but the site of the cyst suggests that it was bronchogenic rather than pericardial in origin.Although the cyst described below may be unique, it has relevance to the aetiology of pericardial cysts in general.CASE EPORT R.D., a 43-year-old electrical engineer, was admitted to hospital in February 1969 for investigation. For the previous two years he had noticed slight breathlessness on climbing stairs and an enlarged cardiac silhouette had been reported on mass miniature radiography. He had been treated for rheumatic fever at the age of 7 years. He smoked 20 cigarettes a day until 1964 and consumed moderate quantities of alcohol.Examination revealed a healthy, heavily built man; there were no abnormal physical signs except an increased jugular 'a' wave and reversed splitting of the second heart sound in the pulmonary area. The pulse rate was regular at 80 beats per minute and the blood pressure was 110/70 mmHg.Routine investigations showed no haematological or biochemical abnormality, and in particular the white cell count, blood urea, protein-bound iodine and serum cholesterol levels were within normal limits.Chest radiographs confirmed the presence of moderate cardiac enlargement without abnormality of the pulmonary vasculature ( Figs. 1 and 2).Right and left heart catheterization demonstrated normal haemodynamics apart from raised end-diastolic pressures in the right ventricle (6-9 mmHg) and in the left ventricle (10-15 mmHg).Cineangiography was performed by injection of contrast medium into the right atrium, showing that all the heart chambers were displaced to the right by a large mass situated on the left lateral aspect of the heart (Fig. 3)