Background: Although, reperfusion therapy for acute myocardial infarction (AMI) by a primary percutaneous coronary intervention (PCI) has been shown to improve the clinical outcome, ischemic heart failure is a major cause of mortality after AMI. The aim of this study was to identify variables that may influence worsening left ventricular function (LVF) in AMI patients who underwent primary PCI. Methods: A total of 96 patients (60.3짹11.7 years, male 85.4%) with decreased LVF [LV ejection fraction (LVEF) < 50%] complicated AMI who underwent primary (PCI) were enrolled. Worsening LVF was defined as decrease of LVEF more than 10% compared with baseline LVEF. The patients were divided into two groups: Group I (recovery LVF, n=76; recovery LVF was defined as improvement of LVEF more than 10% compared with baseline LVEF) and Group II (worsening LVF, n=20). Results: Follow-up (F-U) EF was 58.6짹7.7% in Group I and 32.9%짹6.2% in Group II. More patients had anterior wall MI (50.0% vs. 90.0%, p=0.034) in Group II. The baseline white blood cell (WBC) count, monocyte count, and peak Troponin-T (TnT) level were significantly higher in Group II (11236.8짹3651.5 vs. 15100.0짹6054.8/mm3, p=0.013; 618.9짹283.1 vs. 855.0짹388.1/mm3, p=0.036; 6.9짹5.8 vs. 14.5짹8.4 ng/dL, p=0.002). Postprocedural TnT and C-reactive protein levels were significantly higher in Group II (4.7짹4.1 vs. 13.1짹6.6 ng/dL, p=0.014; 1.1짹0.9 vs. 3.0짹2.5 mg/dL, p=0.005). LVEF (F-U LVEF minus baseline LVEF) correlated with baseline WBC count (r=-0.363, p=0.011), monocyte count (r=-0.304, p=0.036), peak and postprocedural TnT levels (r=-0.456, p=0.002; r=-0.629, p=0.001). There were no significant differences in the incidence of cardiac death and target vessel revascularization between both groups. However, reinfarction was observed 1 patient in group II during clinical F-U. Multivariate analyses showed postprocedural TnT was the independent predictor of the worsening LVF (odds ratio; 12.6, 95% CI; 1.30-23.25, p=0.029) Conclusions: AMI patients who had worsening LVF had more anterior wall MI and postprocedural TnT was an independent predictor of worsening LVE.