Seventy-four children aged 0.3 to 21.4 years (median 4.0) were followed echocardiographically on days 4, 7, 14 and 28 (+/- 2 days) after cardiac surgery to evaluate the incidence of postoperative pericardial effusion, to identify the patients at greatest risk of developing an effusion and to evaluate the use of aspirin as prophylaxis against pericardial effusion. Pericardial effusion was graded relative to the size of the aortic root from grade 0 (no effusion) to grade 5 (larger than the aortic root dimension). Patients were randomly divided into 2 groups: group 1 (32 patients) received aspirin 60 mg/kg/day for 7 days starting on the third postoperative day; group 2 (42 patients) received no aspirin. Forty-eight patients (65%) developed an effusion during the study period, 3 required pericardiocentesis and 1 died of tamponade. All patients with tamponade had a grade 4 effusion. Age or type of operation did not alter the cumulative incidence of significant effusion. No patient with a grade 0 effusion on the first echocardiogram developed a grade 4 or 5 effusion. Results in groups 1 and 2 were similar. Pericardial effusions are common in the first month after cardiac surgery. Patients with no effusion in the immediate postoperative period appear to be at lesser risk of developing a grade 4 effusion in the first month after operation. Finally, aspirin prophylaxis against postoperative pericardial effusions did not significantly alter the outcome in this small series of patients.