The clinical significance of the time of onset of atrial fibrillation (AF) was investigated in patients with acute myocardial infarction (AMI). Among 1,039 patients with AMI, 100 (9.6%) had AF. These patients were divided into 3 groups: AF group 1 (n = 45), who developed AF within 24 hours of the onset of AMI; AF group 2 (n = 41), who developed AF >24 hours after the onset of AMI; and AF group 3 (n = 14), who developed AF before the onset of AMI. The infarct-related lesion was most frequent (67%) in the proximal right coronary artery in AF group 1 (p <0.01). Right atrial pressure was most significantly increased in AF group 1. The left atrial dimension and pulmonary arterial wedge pressure were most significantly increased, and left ventricular ejection fraction was most significantly decreased in AF group 2. In the acute phase, the frequencies of heart failure, cardiogenic shock, and in-hospital mortality were higher for all 3 AF groups than the sinus group (p <0.01). The long-term survival rate was significantly lower in AF group 1 and AF group 2 than in the sinus group. AF was an independent predictor of cardiac death in both AF group 1 (odds ratio 2.5; 95% confidence interval 1.2 to 5.0; p = 0.0012) and AF group 2 (odds ratio 3.7; 95% confidence interval 1.8 to 7.5; p = 0.0005), but not in AF group 3. The onset time of AF appears to be a useful parameter for evaluating the cardiac status and prognosis of patients with AMI.