PURPOSE: Recently, acute myocardial infarction has been reported to show low-attenuated myocardium due to hypoperfusion in contrast-enhanced CT. We studied low-attenuation of myocardium in other conditions than in acute myocardial infarction with multi-detector CT. METHOD AND MATERIALS: Total 655 patients (mean age 61짹10) underwent contrast-enhanced coronary CT angiography (CECT) for evaluation of native coronary artery in suspicion of coronary artery disease (NCA group, n=275) or patency of coronary artery bypass graft (CABG group, n=380) with multidetector CT. They all underwent non-enhanced CT (NECT) with prospective ECG-gated coronary calcium scoring protocol prior to CECT. We analyzed the frequency of low-attenuation of myocardium in CECT and their co-presence in NECT. If low-attenuation region was detected in NECT, it was considered due to fat deposition. The degree of enhancement of low-attenuation area was measured and compared to that of normally enhanced myocardium. RESULTS: Seventy two (11%) patients showed low-attenuation area of myocardium with CECT. With NECT, fat deposition was noted in 61 (85%) patients of them in the same areas with CECT. Sixty patients (15.8%) in CABG group and 12 patients (4.4%) in NCA group showed low-attenuation area of myocardium in CECT. In the 12 patients with low-attenuated area in NCA group, the history of previous myocardial infarction was in 4 patients, but not in 8 (more than 2 vessel disease in 3, and no evidence of ischemic heart disease in 5). CT attenuation value of low attenuated myocardium in NECT and CECT was -24.1짹33.4 and -8.5짹33.9 HU and that of normal myocardium was 44.3짹15.4 and 111.9짹23.9 HU, respectively. The mean enhancement of low-attenuation area was 9.2짹25.8 HU, and that of normal myocardium was 67.6짹15.7 HU. The low-attenuation areas were all confined in subendocardial myocardium except one, and distributed related to vascular territory. CONCLUSIONS: Chronic myocardial infarction or ischemia may show low-attenuation in CECT by either hypoperfusion or fat deposition or by both. Therefore, interpretation of low-attenuation in CECT for diagnosis of myocardial hypoperfusion without NECT should be careful.