Background : Acute myocardial infarction (AMI) at a young age less than 40 years is an uncommon condition and characterized by multiple cardiovascular risk factors. We analysed risk factors of restenosis in AMI patients underwent percutaneous coronary intervention (PCI) under the age of 40 years. Methods : Between January 1997 and July 2006, 80 out of 112 young AMI patients (35짹4 years, 106 male) who underwent follow-up coronary angiography after PCI were divided into two groups: the patients without restenosis (group I: n= 55 patients, 36짹4 years, 55 male) and the patients with restenosis (group II: n= 25 patients, 36짹4 years, 22 male). Clinical and coronary angiographic characteristics were compared between the two groups. Results : Smoking (78.6 %)was the most common risk factor in total patients. Baseline clinical characteristics and baseline laboratory findings were not different between the two groups. Female was more common in Group II. Lesion type according to American College of Cardiology/American Heart Association classification and Thrombolysis In Myocardial Infarction were not different betweeen the two groups. The level of homocysteine (Hcy) was significantly decreased from 12.2짹8.0 關mol/L to 9.0짹3.1 關mol/L in group I (p=0.0009), but not changed significantly in group II (p=0.149). Lipid profiles including low density lipoprotein-cholesterol and triglyceride were improved in both groups. By multiple logistic regression analysis, female sex and the level of Hcy were independent predictors of restenosis. Conclusion : Female sex and high level of homocysteine are predictive factors of restenosis after PCI in young Korean patients with acute myocardial infarction.