Background: Left atrial (LA) enlargement have been known to be a powerful predictor of AF in a wide spectrum of individuals. On the other hand, recent studies show that inflammation may play a role in the pathogenesis of atrial fibrillation (AF). However, the relation of LA volume reflecting diastolic properties before MI and hs CRP presenting acute inflammatory response of MI in early AF in acute myocardial infarction(AMI) have not yet investigated. Method: We prospectively studied 402 patients with AMI from August 2005 to March 2007. All enrolled subjects was further sub-classified into group 1 (321 patients with non-dilated LA; LAVI <32ml/m2) and group 2 (80 patients with dilated LA; LAVI>32 ml/m2. Variables of left ventricular ejection fraction(EF), LA volume index(LAVI), hs CRP, E/E���, diastolic filling patterns, and infarct-related artery were compared between patients with or without early-onset AF developing within 1 day after AMI. Results: Patients who developed early AF were older (p<0.001), and had larger LAVI>32ml/m2 (p<0.001). 17 (5.2%) of 321 patients in group 1 and 16(20%) of 80 patients in group 2 developed early AF. Patients who developed early AF in group 1 were older (p=0.015) and higher hs-CRP tertiles (p=0.001) than those without AF. Patients with early AF in group 2 were older (p=0.02) and had more left circumflex artery in infarct related artery (p=0.043) than without it. By stepwise multivariable logistic regression, the independent predictors of early-onset AF development were age (OR=1.08, 95% CI, 1.03–1.13, p=0.001) and LA volume index (OR=2.83, 95% CI, 1.21–6.62, p=0.016). hs-CRP tertiles( OR=3.02, 95% CI, 1.34-6.77, p=0.007) and age (OR=1.06, 95% CI, 1.013–1.12, p=0.001) are the independent predictors of early AF in group 1, whereas LAVI>32cc/m2 ( left circumflex artery (OR=5.88, 95% CI, 1.39-24.82, p= 0.016) and age(OR=1.06, 95% CI, 1.01-1.25, p=0.026) in group 2. Conclusion: Increased LA volume was a significant predictor of early AF in AMI, whereas hs CRP in AMI with non-dilated LA was an independent predictor of the development of early AF.