The explanation for the high incidence of pleural effusion after cardiac surgery is unclear. There is a high incidence of left pleural effusion with inflammatory pericardial disease. We hypothesized that after coronary artery bypass grafting (CABG) there would be a higher incidence of pleural effusions in patients with pericardial involvement. We prospectively studied 47 patients undergoing elective CABG; 17 had only saphenous vein grafts (SVG group) and 30 received at least one internal mammary artery graft (IMA group) in addition to SVG. Patients had a chest radiograph, chest ultrasound, and an echocardiogram on the 7th, 14th, and 30th postoperative days. Seven days after the surgery, 42 to 47 patients (89.4 percent) had a pleural effusion and 36 (76.6 76.6 percent) pericardial involvement. No relationship was found between the presence of a pleural effusion and a pericardial effusion (p > 0.05). On the 14th postoperative day, 36 patients (76.6 percent) had a pleural effusion while 33 patients (70.2 percent) had a pericardial effusion. There was a significant relationship between the presence of a pleural effusion and a pericardial effusion (p < 0.05). On the 30th postoperative day, 27 patients (57.4 percent) had a pleural effusion and 25 (53.2 percent) had a pericardial effusion. Again, there was a significant relationship between a pleural effusion and a pericardial effusion (p < 0.05). Finally, there was no relationship between the ejection fraction and the presence of pleural effusion at any time (p > 0.05). From this study, we conclude that there is a high prevalence of both pleural and pericardial effusion postoperatively in patients undergoing CABG. Both types of effusions tend to be asymptomatic, gradually disappear, and are more common in the IMA group. Patients who have a persistent pericardial effusion are more likely to have a persistent pleural effusion.