The blood retained in posterior pericardium can trigger an inflammatory response that increases postoperative atrial fibrillations (POAFs), and it can complicate postoperative course. We retrospectively investigated the impact of a posterior pericardial drain (PPD) in reducing late postoperative pericardial effusion (pPE) and POAFs during the first 30 postoperative days. Two hundred and fifty coronary artery bypass grafting patients were divided into two groups according to the presence of a PPD in addition to the anterior one. Perioperative data and the incidence of POAF were compared. Risk factor analysis was used to determine the predictors of pPE and postpericardiotomy syndrome. Late pPE was present in 16% of all patients. It proved to be much more frequent in patients with a posterior drain (odds ratio 2.58; 95% confidence interval 1.23-5.79; P  = 0.015) where it seemed to be almost mild and anterior. 'Anterior Drain' patients showed an increased rate of moderate ( P  < 0.001) and posterior effusions ( P  < 0.001). POAF was much more frequent in patients without a PPD (25.2 vs. 6.3%; P  < 0.001). Univariate risk factor analysis revealed a significant association between late pPE and lower preoperative weight ( P  = 0.003), lower preoperative and postoperative serum albumin ( P  < 0.001) and a greater amount of blood transfusion ( P  = 0.02). Even if a PPD is associated with a higher rate of pPE, the patients with only anterior drains were shown to have a greater amount of pericardial effusion and an increased risk of POAFs. Therefore, a PPD should be considered to improve postoperative course.