Background Wall motion abnormality in the apical legion of the left ventricle (LV) with stagnant flow alone is not sufficient to identify patients at high risk for LV thrombus formation among those with first anterior acute myocardial infarction (AMI). The aim of this study was to identify the determinants of LV thrombus formation using contrast echocardiography. Methods and Results In 75 patients with first anterior AMI, standard and contrast echocardiography was performed to detect LV thrombus. Although LV thrombus was found in 10 patients (13%) using standard echocardiography, it was found in 15 patients (20%) using contrast echocardiography. Apical stagnant flow was observed in 14 patients (93%) with LV thrombus. In addition, patients with LV thrombus had a higher peak C-reactive protein (CRP) concentration (18.2±4.3 vs 7.9±5.5 mg/dl, p<0.0001). In multivariate analysis, only peak CRP concentration was identified as an independent predictor of LV thrombus (p=0.02, odds ratio: 1.400, confidence interval: 1.040-1.884). The receiver-operating characteristics (ROC) analysis revealed the best cutoff value of a peak CRP concentration >10.7 mg/dl to identify patients with LV thrombus (sensivity 0.93, specificity 0.75, area under ROC curve 0.91). Conclusions The peak CRP concentration is a useful marker of patients with first anterior AMI who are at high risk for LV thrombus. (Circ J 2006; 70: 1290 - 1296)