Objectives To analyze risk factors and the relation between clinical,biochemical marker and the stenosis extent of coronary artery in patients below the age of 45 years with acute myocardial infarction (AMI).Methods A retrospective investigation was performed on 92 patients below the age of 45 with AMI at the First Affiliated Hospital of Medical School of Xi’an Jiaotong University in 2003～2007.The etiology,morbidity,risk factors,clinical features and results of coronary angiography were studied.Various clinical and biochemical markers were assessed to find out what were associated with the stenosis extent of coronary artery.Meanwhile,the differences between one-vessel disease (group A)and two-vessel or multi-vessel disease (group B) patients with AMI were comparatively analyzed.Results Risk factors analysis revealed that a history of cigarette smoking,metabolic disorders and abusive drinking were mainly found in young AMI patients below the age of 45 years,and metabolic disorder mainly consists of decreased high-density lipoprotein cholesterol (HDL-C) and hypertriglyceridemia.AMI in patients below the age of 45 years account for 10.3%of all AMI.Angiographically,the incidence of one-vessel affected was most frequent in the young adults (73.75%).The most committed vessel was LAD(80.00% ).A higher incidence with history of hypertension and diabetes or impaired glucose tolerance was found in group B,but a history of preceding angina 1 month earlier was more frequently found in group A.Improved Genisi scores of coronary angiography was lower in group A than in group B (7.49±3.63vs 15.08±6.08).Correlation analysis showed that Iog(LDL-C/HDL-C) (r=0.238,P=0.037),TC/HDL-C(r=0.232,P=0.046) were directly correlated with angiographic scores,and HDL-C(r=-0.202,P=0.042) was inversely correlated.Multielement gradual linear regression analysis showed log(LDL-C/HDL-C),TC/HDL-C were associated with the extent of stenosis of coronary artery.Furthermore,the correlation was linear dependence.Conclusions A history of cigarette smoking,metabolic disorders and abusive drinking are mainly found in young patients with AMI under the age of 45 years.Bad life style is the main motivator.The main cause is atherosclerosis(95%).A higher incidence of history of hypertension and diabetes or impaired glucose tolerance is associated with multi-vessel affected ,log(LDL-C/HDL-C),TC/HDL-C are linear dependent on the extent of stenosis of coronary artery.Thus changing of bad life style(stop smoking et al),improving of lipoprotein profile are more important for the prevention and treatment of AMI in young adults.