Non-electrocardiogram-gated contrast-enhanced computed tomography (non-ECG-gated CT) is available in most hospitals where patients with chest and/or back pain are admitted to the emergency department. Although it has been established as the initial diagnostic imaging modality for acute aortic dissection (AAD) and pulmonary thromboembolism (PE), its diagnostic ability for acute coronary syndrome (ACS) in the emergency department has not been elucidated. We retrospectively investigated 154 consecutive patients who required non-ECG-gated CT to differentiate AAD and PE in the emergency department, but had no evidence of them on CT. Furthermore, a subanalysis was performed in the patients who were subsequently suspected of ACS and underwent emergent invasive coronary angiography followed by CT. We evaluated left ventricular enhancement to detect myocardial perfusion deficit by calculating Hounsfield units, and the results were compared with ultimate diagnoses and angiography findings. A perfusion deficit was detected in 43 patients, among whom 26 were ultimately diagnosed with acute myocardial infarction (AMI); 24 patients required emergent revascularization. The subanalysis indicated that perfusion abnormalities corresponded with the territory of the culprit artery in all except one patient. In the remaining 111 patients without perfusion deficit, only two required emergent revascularization, and their levels of creatine kinase MB were not elevated. The sensitivity, specificity, and positive and negative predictive values of non-ECG-gated CT in predicting AMI/emergent revascularization were 93 %, 87 %, 61 %, and 98 %/92 %, 85 %, 56 %, and 98 %, respectively. Non-ECG-gated CT facilitates the diagnosis of ACS and the decision on emergent catheterization, providing information on the ischemic myocardial area by detection of a localized decrease in left ventricular enhancement.