Background: Insulin resistance (IR) is an associated with a significant increase in the risk of coronary artery disease. The serum Apolipoprotein B (Apo B) and Apo B/Apo-I ratio are important markers of IR in non-diabetic, normoglycemic subjects. We investigated the role of Apo B and Apo B/Apo A1 ratio in patients with acute Non-ST elevation myocardial infarction (NSTEMI) with total occlusion (TO) of infarct-related artery (IRA). Methods: A total of 280 patients (64.5짹12.7 years, male 70 %) with acute NSTEMI who were underwent percutaneous coronary intervention were enrolled. We excluded patients with history of diabetes, or medication of oral hypoglycemics, or hemoglobin A1c (HbA1c) more than 6.5%. The patients were divided into two groups according to the finding of IRA (Group I: patients with TO of IRA, n=84; Group II: patients with non-TO of IRA, n=196). Results: The level of troponin I are significantly higher in Group I than in Group II (83.3짹30.1 vs. 71.6짹33.7 ng/mL, p=0.044). Most frequent IRA was left circumflex artery in Group I (50%) and Group I had more complex vascular lesion compared with Group II (Type C lesion 64.3% vs. 29.0%, p<0.001). The level of Apo B and Apo B/Apo A1 ratio were significantly higher in Group I than in Group II (108.1짹25.3 vs. 96.3짹25.4, p=0.019; 0.87짹0.21 vs. 0.74짹0.22, p=0.004, respectively Group I was divided into two subgroups according to initial therapeutinc strategy. In subgroup analysis, early invasive treatment group had lower in-hospital adverse events than in early conservative treatment group (5.7% vs. 10.1%, p=0.046). ). In multivariate analysis, elevated Apo B/Apo A1 ratio showed high probability of TO of IRA (odd ratio, 3.56; 95% CI, 1.12 to 11.38, p=0.031). Conclusions: The level of Apo B and Apo B/Apo A1 ratio were associated TO of IRA in patients with acute NSTEMI. The markers may be useful for choice of initial therapeutic strategy in acute NSTEMI.