Background: Pericardial effusion is commonly seen after coronary arterybypass surgery; large pericardial effusion develops at 4 to 10 days postoperatively in 30% of patients undergoing cardiac surgery. However, regional pericardial effusionsare often localized posteriorly; one of the recent solutions to reduce this complication is posterior pericardiotomy, which also significantly reduces latepericardial effusion and late posterior tamponade. Aim: our target is to assess the impact and effectiveness of posterior pericardiotomy in reducing post-cardiac surgery pericardial effusion and tamponade. Patients and Methods: This prospective randomized case-controlled study was carriedout in 64 patients. They were randomly assigned to one of 2 groups of32 patients each. Longitudinal incision in the pericardiumwas made parallel and posterior to the left phrenic nerve in group one and not in group II, then we evaluated the results between the two groups as regards post operative pericardial effusion. Results: There were no statistically significant differencesbetween the groups with respect to demographic or operativecharacteristics. The pericardiotomy group had lower incidence of postoperative pericardial effusion as 68.75% of the Pericardiotomy group had no effusion in the first seven days, while it was 43.3% in the control group, and none of the Pericardiotomy group developed severe pericardial effusion at any time. Conclusion: posterior pericardiotomy significantly reduce postoperative effusion and tamponade, and we recommend it in all the cases of cardiac surgery operations.