Background: Although obesity is a known risk factor of coronary artery disease, outcome after revasculization has been shown to have better in obese patients. This study was aimed to examine the possible mechanisms between obesity and outcome in Korean acute ST segment elevation myocardial infarction (STEMI). Methods: 3444 eligible STEMI patients (2536 males, 64.1짹12.8 years) enrolled in Korea Acute Myocardial infarction Registry (KAMIR) were classified into 4 groups by WHO body mass index (BMI) criteria: underweight (<18.5 Kg/m2), normal weight (18.5-24.9 Kg/m2), overweight (25-29.9 Kg/m2), obesity (���30 Kg/m2). Results: Overweight/obese patients were 33.8% in STEMI patients. Invasive treatment was more commonly done with the increasing BMI (77.0% vs. 81.8%. vs. 85.1% vs. 87.8%, p<0.001). Multivariate logistic regression analysis showed that overweight/obesity (OR, 1.273; CI, 1.030-1.572; p=0.025), old age (OR, 1.327; CI, 1.059-1.662; p=0.014) were the strongest determining factors in invasive strategy. Overweight/obese patients were younger (p<0.001), had lower Killip class (p<0.001), cardiogenic shock (p<0.001), and hs-CRP (p=0.033). However, major in-hospital events (death, MI, cardiovascular event) (OR, 0.521; CI, 0.375-0.724; p<0.001), in-hospital cardiopulmonary events (OR, 0.745; CI, 0.602-0.921; p=0.006) were less common in overweight/obese patients. Overweight/obese patients had lower 1-month major adverse cardiac events (MACE) (OR, 0.745; CI, 0.583-0.953; p=0.019) and mortality (OR, 0.492; CI, 0.346-0.699; p<0.001). One-year MACE (OR, 0.337; CI, 0.170-0.671; p=0.001) and mortality (OR, 0.070; CI, 0.009-0.517; p=0.001) were lower in overweight/obese patients. Conclusions: Better prognosis in overweight/obese patients with STEMI is related with initial invasive strategy, less in-hospital complication, and less inflammation.