Purpose: To assess the usefulness of multidetector helical CT (MDCT) in imaging coronary arteries and in detection and sizing of acute and chronic myocardial infarction (MI) in comparison with MR imaging. Materials and Methods: 46 patients underwent MDCT and MRI in the acute stage (within 2 weeks, n = 27) and chronic stage (6-12 months, n = 19) of MI. First-pass and 5-minutes and 15-minutes delayed myocardial MR imaging was performed using an 1.5 T scanner with injection of Gd-DTPA to assess the extent of MI. Within 24 hours after MR imaging, ECG-gated MDCT was performed using a 4-slice or 16-slice scanner at 25-seconds and 10-minutes delay. Short-axial CT images of 2 mm slice thickness were reconstructed from the CT data. Two radiologists analyzed the perfusion defects and hyperenhanced area on MR and CT images and calculated the percentage of lesion to total left ventricle wall area. The calculated values of the two observers were averaged and statistical analysis was performed for comparison of two modalities. Results: MDCT showed patency of infarct-related arteries including stented segments in all cases. In most cases, MDCT showed lesions of MI on early (97.8%) and late phase images (100%). The percent area of the lesion on MDCT images (early and late phases) correlated well with that of MR images (perfusion and delayed images). The correlation coefficient was 0.75 between perfusion MR and early CT, and 0.84 between 5-minutes delay MR. Conclusion: Myocardial perfusion imaging as well as coronary artery imaging is feasible with MDCT. The size of MI on early phase MDCT is significantly larger than that of MR imaging.