To evaluate the incidence and clinical significance of infarction-associated pericardial effusion in patients with successful primary percutaneous transluminal coronary angioplasty, we studied 214 consecutive patients with a first Q-wave acute myocardial infarction. Based on 9 clinical variables, multivariate analysis was performed to determine the important variables related to the occurrence of pericardial effusion. Pericardial effusion was detected by echocardiography in 45 patients (21%); pericardial rub (p <0.001), number of advanced asynergic segments (p <0.001), ventricular aneurysmal motion (p = 0.03), and pulmonary capillary wedge pressure (p = 0.04) were found to be the important variables related to pericardial effusion. Among 45 patients with pericardial effusion, 29 patients with no pericardial rub had significantly higher pulmonary capillary wedge pressure than those with pericardial rub, whereas 16 patients with pericardial rub had a higher incidence of angiographic no reflow and ventricular aneurysmal motion than those without pericardial rub. Patients with pericardial effusion and a pericardial rub had a higher mortality rate than those without pericardial effusion (19% vs 3%; p = 0.02). Thus, pericardial effusion is still a relatively common clinical finding after primary percutaneous transluminal coronary angioplasty, and those with pericardial effusion and a pericardial rub were associated with more severe transmural myocardial damage and higher in-hospital mortality.