Backgrounds: Recent data suggested that low body mass index (BMI) was one of the risk factor in clinical outcomes after acute myocardial infarction (AMI), as BMI paradox. This study aimed to know how BMI effects on early outcomes after acute ST elevated MI (STEMI), especially, in the patient underwent primary percutaneous coronary intervention (PCI). Methods: As part of KAMIR study, this study enrolled 2590 STEMI patients who admitted to hospital within 12 hours and underwent primary PCI, from December 2005 to December 2006. Patients were stratified into three categories as follow; Group I: BMI<25 (n=1640), Group II (overweight): 25���BMI<30 (n=862), Group III (obesity): BMI���30 kg/m2 (n=88). Major adverse cardiovascular events (MACE) including death, recurrent MI, angina pectoris and revascularization rate were analyzed and compared among each groups. Results: In baseline characteristics, initial left ventricular ejection fraction was decreased with declining BMI (p<0.001). There were no significant differences in the number of diseased vessels, lesion type and initial TIMI flow. During hospitalization, total death rate was higher in low BMI group (p=0.016) and statin was more prescribed in high BMI patients (p=0.029). At 6th month on follow up, total death, cardiac death and total MACE were higher in low BMI group than high BMI group (p=0.009, p=0.005, p=0.037, respectively). But, multivariate analysis revealed that BMI had no significance as risk factors in total death, cardiac death and total MACE after adjusting for other confounding factors. Conclusion: This study showed no impacts of BMI on early outcomes in STEMI patients who underwent primary PCI. There was no BMI paradox in our study.