Patients with acute coronary syndrome require accurate diagnosis and treatment. We found that emergency myocardial scintigraphy with 99mTc-tetrofosmin was useful for diagnosis in 2 patients with acute coronary syndrome. The first patient was a 66-year-old man, who was hospitalized with chest discomfort at rest. Blood tests, electrocardiography, and echocardiography revealed no unusual findings. Myocardial scintigraphy with 99mTc-tetrofosmin disclosed moderately reduced uptake extending from the anterior wall to the apex. Coronary arteriography revealed 75% stenosis of the left main trunk. He was diagnosed as having unstable angina pectoris and coronary bypass surgery was performed. The second patient was a 61-year-old man, who was hospitalized with chest discomfort. Slight leukocytosis and flat T waves in leads V5 and V6 on electrocardiogram were detected, but echocardiographic findings were not abnormal. Myocardial scintigraphy with 99mTc-tetrofosmin revealed a defect in the posterolateral wall. Coronary arteriography showed total obstruction of #14, and a diagnosis of acute myocardial infarction was established. PTCA of #14 was performed to achieve reperfusion.