Objective: Pericardial effusion (PE) after cardiac surgery is frequent. It is more frequently seen after valve replacement or other types of heart surgery. Oral anticoagulants and antiplatelet agents may induce effusion development after open heart surgery. Our objective was to determine the efficiency of posterior pericardiotomy (PP) after cardiac valve operation for reducing the incidence of early and late PE and tamponade. Methods: This prospective randomized study was carried out in 100 consecutive patients undergoing mechanical valve replacement between August 2001 and May 2003 in our institution. Patients were divided into two groups; each group consisted of 50 patients. Longitudinal incision was made parallel and posterior to the left phrenic nerve, extending from the left inferior pulmonary vein to the diaphragm in Group 1. Posterior pericardiotomy was not done in Group 2. Results: Early PE was detected in four patients (8%) and in 19 patients (38%) in Group 1 and Group 2, respectively (p < 0.001). No late PE effusion was developed in Group 1 despite nine (18%) late PE developing in Group 2 (p < 0.003). The rate of delayed pericardial tamponade was lower in Group 1, but this difference was not statistically significant (0% vs 10%; p < 0.056). Conclusion: These findings suggest that PP is an easy, feasible, and beneficial technique for reducing both the occurrence of early and late PE or pericardial tamponade in patients undergoing valve replacement.