Persistent pericardial effusion in the absence of obvious preceding or concurrent disease presents a baffling problem. In such cases chylopericardium, cholesterol pericarditis, lupus erythematosus, malignant disease, myxedema, severe anemia, and scleroderma, must be considered. Six cases are reported with large effusions present for from 1 to 11 years. The diagnostic measures to be used, the results of histologic examination, and the effects of surgery on the clinical course are discussed.