The patient was a 64-year-old woman with systemic lupus erythematosus, thrombophlebitis of the lower legs, cerebral infarction with left hemiparesis, and colostomy after perforation of the sigmoid colon. On the morning of her presentation, the patient felt epigastric abnormality. Thereafter, hematemesis occurred twice, leading her to call an ambulance in the afternoon. Upon arrival, electrocardiography before securing a venous route and obtaining blood samples revealed ST segment elevation in leads II, III, and aVF. As her vital signs were stable and her hemoglobin level had decreased by just 1.1 g/dl in comparison to the previous day, emergency coronary angiography (CAG) was performed. CAG revealed complete occlusion at section #4. She underwent right coronary angioplasty with stent placement. The patient's course after angioplasty was uneventful. On the 15th hospital day, esophagogastroduodenoscopy revealed esophageal erosion and superficial gastritis. She was discharged on foot the following day. When physicians treat patients with hematemesis, electrocardiography and the measurement of troponin are essential before esophagogastroduodenoscopy.