Large pericardial effusions develop in 30% of patients after cardiac surgery, and reach their maximum size after 10 days, with tamponade in 1%. The aim of this prospective randomized case-controlled study was to assess the effectiveness of a posterior pericardiotomy in preventing early and late (>30 days) development of pericardial effusion. Between April 2005 and May 2006, 410 patients with a mean age of 68.4 +/- 9.2 years undergoing coronary artery bypass grafting alone or combined with valve surgery were divided into 2 groups of 205 each. In the pericardiotomy group, a 4-cm longitudinal incision was made parallel and posterior to the phrenic nerve. Echocardiography was performed at discharge and 15 and 30 days after the operation. At 15 and 30 days postoperatively, 90.2% and 97% of patients in the pericardiotomy group were free of effusion; while none in the control group were free of effusion. A posterior pericardiotomy is easy to perform and seems to be a safe and effective means of preventing postoperative effusion and its adverse consequences.