Objective: In this study, we aimed to determine the causal relationship between isolated coronary bypass surgery development of severe pleural effusion in the early postoperative period. Methods: In this study, 7862 isolated coronary artery bypass surgery cases conducted in our hospital between February 2001 and July 2013 were analyzed retrospectively. 175 (2.2%) patients developed early-severe pleural effusion were included in the study. The findings of 175 cases with early severe postoperative pleural effusion (Group A) were compared with the data of 180 cases (Group B) who had similar demographic findings and did not develop early-serious pleural effusion in the postoperative period. Results: The mean age of these pleural effusion cases was 66.3±9.5 (55-76). 149 (85.1%) of the cases were operated under elective conditions and 26 (14.9%) of them were operated under emergency conditions. The mean cardiopulmonary bypass time of the cases was 70±28 (40-100) minutes. Left internal thoracic artery (LITA) and saphenous vein graft (SVG) were used together in 161 (92%) cases. Patients discharged after operation uneventfully. However, of the cases, 126 (72%) had shortness of breath, 115 (65.7%) dry cough, 28 (16%) productive cough, 50 (28.6%) tachypnea, 22 (12.5%) chest pain. Early severe pleural effusion developed in the left hemithorax in 154 (88%) of the cases. LITA+SVG was used in 148 (96.1%) of these cases, and only SVG was used in 6 (3.9%) cases. Early severe pleural effusion developed in the right hemithorax in 14 (8%) of the cases. LITA+SVG was used in 10 (71.4%) of these cases, and only SVG was used in 4 (28.6%) cases. The cases with bilateral early severe pleural effusion were 7 (4%). LITA+SVG was used in 3 (42.9%) of these cases, and only SVG was used in 4 (57.1%) cases. When the two groups were compared, Group A had significantly higher rates of low EF and atrial fibrillation than Group B. Conclusion: It is known that coronary artery bypass surgery causes deterioration of postoperative pulmonary function. One of the most common complications of coronary artery bypass surgery is pleural effusion. It is known that this postoperative picture is associated with increased hospital readmission, rehospitalization and high postoperative morbidity. It is important to determine preoperative risks in terms of postoperative patient management and morbidity assessment.