Pericardial effusion frequently occurs after cardiac operation. Despite its high incidence, the etiological process of postoperative pericardial effusion remains unclear. Residual blood or thrombus has often been suggested as a possible cause, implying that the occurrence of pericardial effusion could be related to the effectiveness of postoperative thoracic drainage. This possible relationship, however, has never been studied. We found that prolonging the duration of thoracic drainage by 24 hours often increases total chest tube output considerably but does not affect the incidence of postoperative pericardial effusion: approximately 55% of 100 patients in this study were shown by two-dimensional echocardiography to have pericardial effusion on the sixth postoperative day, regardless of the duration of postoperative drainage. Because of this, and because a long period of drainage causes discomfort for the patient, mechanical irritation to the heart and the pericardium, and an increased risk of infection, we recommend removing drains as soon as their efficacy has peaked, preferably on the first postoperative day.