In order to evaluate the present incidence of post-pericardiotomy syndrome (PPS) and the reliability of conventional diagnostic methods in its detection, 194 consecutive patients were studied by echocardiography one month after cardiac surgery and thereafter every third month up to one year, and also during hospitalization if symptoms or signs of PPS appeared. PPS was diagnosed on the basis of pericardial effusion demonstrated by echocardiography and was found in 15 patients (8%) 7-142 days (mean 44 days) after operation. Four of these patients (27%) were without symptoms. In 7 patients (47%) there were no diagnostic signs on the electrocardiogram and the chest X-ray and in 6 of those patients the pericardial friction rub was missing. Hence in 40% of the patients echocardiography was the only way to make correct diagnosis. In the investigation of a patient with chest pain and/or fever after cardiac surgery, echocardiography seems very useful since the conventional electrocardiogram and the chest X-ray are often unreliable because of the changes caused by the cardiac surgery per se.