Background and Objectives: In early phase of acute chest pain, diagnosis of acute coronary syndrome (ACS) is often difficult in emergency department (ED) due to non-diagnostic ECG and cardiac markers. Ischemia modified albumin (IMA) has recently been shown to be an early sensitive biochemical marker of ischemia. The aim of this study was to evaluate the diagnostic value of IMA in patients with suspected ACS and non-diagnostic EKG/biochemical markers. Subjects and Method: We enrolled consecutive 142 patients presenting to the ED due to suspected ACS with non-diagnostic EKG and negative troponin-I/CK-MB in 5 hours of chest pain. The diagnosis of ACS was based upon clinical findings, results of serial ECG/troponin and coronary angiography. The ideal cutoff value of IMA for ACS was calculated by Receiver Operator Characteristic (ROC) curve analysis. Results: The ACS was diagnosed in 80/142(56%). The ROC curve area for the IMA test was 0.77 (confidential interval, 0.70-0.85, p<0.01). At a cutoff value of 98.5 U/mL, the sensitivity, specificity and negative predictive value for ACS were 73%, 75% and 75%, respectively. At a cutoff value of 85 U/mL, the sensitivity, specificity and negative predictive value for ACS were 92%, 35% and 95%, respectively. Conclusions: IMA might be a useful diagnostic marker of ACS in patients with non-diagnostic EKG/necrosis marker presenting within 5 hours after onset of chest pain.