Echocardiography (echo) is frequently performed postoperatively to evaluate patients suspected of having cardiac tamponade or pericarditis. The overall incidence and significance of echocardiographic pericardial effusions (PE) early after cardiac surgery are unknown. Therefore, M-mode and 2-dimensional (2-D) echo were used to study 39 stable patients 4 to 10 days after cardiac surgery. Twenty-two patients (56%) had unequivocal moderate-to-large PEs. PEs were identified on serial chest x-rays in only 6 patients. PEs were significantly more common after heavy postoperative bleeding, and occurred in 16 of 19 patients with more than 500 ml of total chest tube output; only 6 of 20 patients with chest tube output less than 500 ml had PE. There was no correlation of PE by echo with pericardial friction rubs, chest pain or atrial arrhythmias. Elevated erythrocyte sedimentation rate did not correlate with PE by echo or clinical pericarditis. In 1 of 22 patients with PE, tamponade developed, and the patient required reoperation on day 5; the other 21 were discharged without related therapy. Thus, early postoperative PEs are common and related to postoperative bleeding. Because they do not correlate with symptoms of pericarditis and rarely lead to tamponade, their identification is usually of limited clinical significance.