The frequency and clinical outcome of pericardial effusions was investigated in 104 patients following open-heart surgery. A pre- and post-operative echocardiogram was made in all cases. Pericardial effusions were present in 26%, significantly more occurring after bypass surgery (37%) than after valve replacement (15%). In 4 out of 5 cases the effusion had no haemodynamic effect and in no instance was a repeat thoracotomy necessary. A significantly higher incidence of reduced exercise tolerance, increased heart rate, atrial fibrillation and a radiologically demonstrable increase in heart size of more than 10 mm, were indicative of the presence of pericardial effusions. In contrast, central venous pressure and blood pressure were no higher than in patients without pericardial effusion. Complication-free pericardial effusions thus occur frequently after cardiac surgery. A reliable diagnosis is only possible by means of echocardiography. It remains to be shown whether the long-term clinical progress is affected.