Background : Acute myocardial infarction (AMI) at a young age less than 40 years is an uncommon condition and it is characterized by multiple cardiovascular risk factors. We analyzed the risk factors of Korean AMI patients in young age(younger than age of 40) and the other AMI patients ,who were registered in Korea Acute Myocardial Infarction Registry (KAMIR). Subjects and Methods : In the year of 2006, 8565 patients (mean age: 64.4짹12.7 years, 5591 males) were registered in KAMIR, were divided into two groups: the patients in young age(���40)(group I: n=261, mean age: 35.9짹4.5 years, 245 males) and the patients in other age group(group II: n=8304, mean age: 65.4짹11.8 years, 5330 males). The clinical and angiographic characteristics and MACE were compared between the two groups. Results : In baseline clinical characteristics, there were many differences between two groups, especially in gender, age, risk factors (hypertension, smoking, diabetes, familial history), body weight and GFR (p<0.001). But there was not significant difference in hyperlipidemia. Also, they have different early outcomes, like CCU days, complications and mortality(p=0.001, 0.036, <0.001). Baseline laboratory findings were different between the two groups, in initial EF, CK, CK-MB, Tn-I (p=0.008), TC, TG, and NT-proBNP(p=0.001)(others, p<0.001), but not in hs-CRP, Tn-T, HDL and LDL-cholesterol. Also there were significant difference in coronary angiographic findings (IRA, vessel number, TIMI flow). Group I preferred sirolimus eluting stent rather than paclitaxel eluting stent (p=0.005). 5590 patients had 6-month clinical follow up, the 6-month MACE were significantly lower in group I (p=0.016). According to multiple logistic regression analysis, body weight, diabetes mellitus, familial history and triglyceride level were independent predictors of young age AMI (p<0.001, 0.001, 0.047, 0.001). Conclusion : The patients group with less than 40 years of age and more than 40 years of age have many significant difference with each other. The independent predictors of young age AMI were body weight, presence of diabetes mellitus, familial history of myocardial infarction and high serum level of triglycerides.