Abstract Background: Pericardial window (PW) is a technique that allows the passage of fluid from the pericardial to the pleural cavity to reduce the postoperative pericardial effusion. The purpose of this study was to evaluate the effectiveness of the pericardial window in decreasing pericardial effusions after cardiac surgery. Methods: The study included 400 adult patients who underwent cardiac surgery from 2017 to 2020. Patients were randomly assigned into two groups; the pericardial window (PW) group included 200 patients who underwent posterior pericardiotomy, and the control group included 200 patients who did not undergo this procedure. Results: Preoperative data were comparable between both groups. More patients in the PW group had chest tube drainage more than 500 cc/ 24 hours (40 (20%) vs. 5 (2.5%), respectively; p=0.005). The drainage of 500 cc/24 hours or more in the mediastinal tube was lower in the PW group (10 (5%) vs. 40 (20%) patients in the PW and control groups, respectively; p