BACKGROUND Acute coronary syndrome is life-threatening. The diagnosis can be confirmed by electrocardiography (ECG) and serum cardiac biomarkers. Early diagnosis and treatment of non-ST segment elevation myocardial infarction (NSTEMI) is important because delayed treatment is associated with poor prognosis, especially in older adults. CASE REPORT An 82-year-old woman presented to the Emergency Department (ED) with epigastric and back pain. Despite the symptoms, the electrocardiogram revealed no abnormality, and the high-sensitivity cardiac troponin (Hs-cTn) value was below the detection limit. Chest contrast-enhanced computed tomography (CT) performed to exclude fatal diseases such as aortic dissection revealed no obvious abnormalities. The patient's symptoms improved and she was discharged. On the following day, the radiologist reviewed the CT and noted reduced cardiac uptake of contrast medium, and so suspected a subendocardial infarction. The patient was immediately recalled to the ED. She had no symptoms, but her Hs-cTn level was markedly elevated and asynergy was found on echocardiography. Emergency coronary angiography revealed complete obstruction of the left anterior descending coronary artery. CONCLUSIONS Similar to patients with acute ST segment elevation myocardial infarction, those with unstable angina or NSTEMI should be treated early. Delayed diagnosis and treatment of acute coronary syndrome is associated with poor prognosis, especially in older adults. Therefore, in patients presenting to the ED with chest pain, careful attention should be paid to myocardial staining in addition to the aorta, pulmonary arteries, and abdominal organs, when performing contrast-enhanced CT.