Seventy one cases of apparently idiopathic chronic pericardial effusion were systematicaly followed up. There were 3 inclusion criteria: patients had to be symptomatic with radiological cardiomegaly without tamponade or an acute onset; the effusion had to be stable for more than 3 months despite medical therapy at the time of surgery; the parietal pericardium removed at surgery had to be of normal histopathological appearances. Cases with a discernable classical cause of pericardial effusion were excluded: previous mediastinal radiotherapy, connective tissue diseases, malignancies, chronic infection or a recent episode of acute pericarditis. There were 9 deaths during the first five postoperative years, one of which was related to the pericardial disease and surgery. By definition, no cause was found but in one third of cases a relationship between the pericarditis and other disease processes was possible. The management of these large pericardial effusions resistant to treatment without any apparent underlying pathology and in which echocardiography only shows a large effusion should be surgical with drainage of the pericardial cavity, allowing exclusion of aetiologies which are non-identifiable by other methods, with a negligeable operative risk even in elderly patients and good long-term clinical results. This recommendation is based on systematic follow-up of over 5 years in 57 of our 71 cases; after drainage, only one of the nine deaths observed was related to the pericardial disease itself and 59, subjects (83%) had no further symptomatic pericardial disease.