Background: Imaging studies to determine myocardial infarct size (IS), such as MRI and nuclear testing, are difficult to perform in an emergency setting. Delayed enhanced (DE) MDCT has recently emerged as a viable alternative available on an urgent basis. We evaluated the feasibility and reliability of DE MDCT in determining IS in patients with acute ST-elevation myocardial infarction (STEMI) who underwent primary percutaneous coronary intervention (PCI). Methods: Twenty consecutive patients (20 men, mean age 58짹9.0 years) presenting with first acute STEMI within 12 hours of symptom onset underwent DE MDCT immediately after successful primary PCI without injection of an additional contrast media. IS was calculated as the total volume of myocardium showing delayed enhancement. The extent of initial ST-segment elevation (STE) and serum levels of cardiac biomarkers (before PCI and serially up to 36 hours) were determined. Results: The mean symptom-to-door and door-to-balloon times were 158.8짹140.5 and 82.1짹49.3 minutes, respectively. The total volume of contrast media used during PCI was 164.7짹39.4mL, and the delay between the last contrast injection and the CT scan was 15.5짹8.0 minutes. Manual thrombectomy was performed in 7 patients and a glycoprotein IIb/IIIa inhibitor was used in 11 patients. The culprit vessel was the left anterior descending artery in 13 patients, right coronary artery in 6 patients, and left circumflex artery in 1 patient. The rates of pre-PCI TIMI 0-1 and post-PCI TIMI 3 were 80% and 100%, respectively. The mean IS by DE MDCT was 24.7짹16.8mL and the mean left ventricular ejection fraction (LVEF) was 57.0짹10.8%. The initial peak STE was 4.2짹2.3 mm, and the peak serum levels of creatine kinase (CK), creatine kinase-MB (CK-MB), and troponin-I were 2,789짹1,622 U/L, 179짹115 U/L, and 107짹84 ng/mL, respectively. Serum levels of high-sensitivity C-reactive protein (hs-CRP) and N-terminal pro-B type natriuretic peptide (NT-proBNP) at baseline and 24 hours after PCI were 0.41짹0.97 mg/dL and 1.38짹1.38 mg/dL, and 189짹253 pg/mL and 1,246짹1,015 pg/mL, respectively. The IS significantly correlated with the peak STE (r=0.688, p=0.003) and the peak levels of CK (r=0.747, p=0.001), CK-MB (r=0.641, p=0.007), and hs-CRP at 24 hours (r=0.569, p=0.027). A significant negative correlation was found between IS and LVEF (r= - 0.682, p=0.004). Conclusion: DE MDCT immediately after primary PCI for acute STEMI is effective in assessing myocardial IS without the need for an additional contrast media injection.