To elucidate the role of inflammatory and hemodynamic factors in the genesis of atrial fibrillation in acute myocardial infarction, 228 patients with a first Q wave anterior myocardial infarction were studied. Forty-nine patients had pericarditis (detection of pericardial rub by careful auscultation), and 36 patients had echocardiographically demonstrated hydropericardium (presence of pericardial effusion without pericardial rub). During the first 3 days after admission, transient episodes of atrial fibrillation were observed in 10 patients (20%) with pericarditis (group 1), 15 patients (42%) with hydropericardium (group 2), and 20 patients (14%) without pericarditis and hydropericardium (group 3). Although there was no significant difference in the incidence of atrial fibrillation between groups 1 and 3, patients in group 2 had a significantly higher incidence of atrial fibrillation than those in groups 1 and 3. Pulmonary capillary wedge pressure and the number of advanced asynergic segments were found to be the important factors discriminating the three groups by multivariate analysis. Therefore atrial fibrillation after acute Q wave anterior infarction was not related to the inflammatory infiltration involving the atria but to the increase in atrial pressure resulting from hemodynamic change caused by more extensive myocardial damage.