Background: This study investigated the relationships between body mass index (BMI) and improvement of left ventricular (LV) systolic function and clinical outcomes in patients with acute ST-segment elevation myocardial infarction (STEMI). Methods: A total of 834 consecutive patients (62.8짹12.0 years, male 75.3 %) with acute STEMI and LV ejection fraction (EF) < 50% who were registered in Korea Acute Myocardial Infarction Registry from Nov. 2005 to Dec. 2007 were enrolled. All patients underwent percutaneous coronary intervention (PCI). The follow-up (F-U) LVEF was measured at 6 months after PCI. The patients were divided into three groups according to BMI [group I: BMI < 23 kg/m2 (underweight or normal body weight), n=280; group II: BMI ��� 23 kg/m2, (group IIa: BMI 23.0-27.4 kg/m2 (overweight), n=468; group IIb: BMI ��� 27.5 kg/m2 (obese), n=86)]. Results: Group I was the oldest and had the lowest glomerular filtration rate (GFR). The level of N-terminal pro-B-type natriuretic peptide (NT-proBNP) was the highest in the group I (3448.3짹7374.3 pg/mL vs. 1759.5짹4079.1 pg/mL vs. 1304.5짹2408.7 pg/mL, p=0.001). At 6-month after PCI, the LVEF was improved significantly in the group I and the group IIa but not in the group IIb (40.3 % to 51.7%, p<0.001; 42.7% to 47.6%, p<0.001; 40.4% to 42.8%, p=0.102). F-U LVEF was the highest in the group I (51.7% vs. 47.6% vs. 42.8%, p<0.001). At 1 month, the composite of major adverse cardiac events (MACEs) were not different in the groups. However, the incidence of cardiac death was higher in the group I than in the group II (0.5 % vs. 0.0 %, p=0.049). At 6 months, the rate of total MACEs and cardiac death were higher in the group I than in the group II (14.2% vs. 10.6%, p=0.043; 1.9% vs. 0.5%, p=0.008). At 12 months, the rate MACEs was not different in the groups. BMI was negatively correlated with age, NT-proBNP level, and delta LVEF (F-U LVEF minus baseline LVEF) (r=-0.241, p<0.001; r=-0.159, p<0.001; r=-0.388, p<0.001, respectively). In multivariate analysis by logistic regression, BMI was an independent predictor of LV functional improvement (OR, 5.41, 95% CI 4.25-6.88, p<0.001). In survival analysis, improvement of LV function was associated of lower incidence of death/MI (HR=0.38, p=0.038). The level of NT-proBNP (OR, 1.64, 95% CI 1.12-2.41, p=0.011) was associated with clinical outcomes in patients with STEMI but not BMI (p=0.328). Conclusion: BMI was an independent predictor of LV functional improvement in patients with STEMI. However, the lower BMI had, the worse clinical outcomes had in spite of LV functional improvement. NT-proBNP was more important prognostic factor in patients with STEMI compared with LV functional improvement.