To determine the incidence and clinical significance of pericardial effusion after acute myocardial infarction, two-dimensional echocardiography was serially performed in 137 consecutive patients. Pericardial effusion was observed in 45 patients (33%), of whom 22 were followed until they recovered and were discharged. Pericardial effusion was more frequent in patients with anterior acute infarction than those with inferior acute infarction, and so it was in non-recanalized patients than in recanalized ones. Patients with pericardial effusion had higher peak levels of creatine kinase, higher wall motion score indices, and higher defect scores of thallium imagings. The improvement of regional wall motion at an infarct zone in patients with pericardial effusion was less regardless of the successful early recanalization. These results show that pericardial effusion is a common event in patients with acute myocardial infarction and observation of transition of pericardial effusion is important for predicting prognosis.