To elucidate the genesis and effect of atrial fibrillation (AF), 102 patients with acute myocardial infarction were studied. Eighteen patients had AF during the first 72 hours in the coronary care unit. The hospital mortality rate was 23%. Discriminant analysis was used to determine the important variables contributing to the genesis of AF and hospital mortality based on the following variables: cardiac output, pulmonary capillary wedge pressure, right atrial pressure, systolic blood pressure (at admission and before the onset of AF or most abnormal value), age, location of infarction, sex and pericarditis. Pulmonary capillary wedge pressure, right atrial pressure and age were the important factors contributing to AF, whereas pulmonary capillary wedge pressure and age were important for hospital mortality. Therefore, the hemodynamic change imposed on the left atrium and aging are the major factors related to the occurrence of AF and hospital mortality.