Background: We aimed to evaluate the perfusion index (PI) level at the 10th minute of admission to the emergency department in patients with non-ST-segment elevation myocardial infarction (NSTEMI) and unstable angina pectoris (USAP). In addition, we aimed to evaluate whether PI is useful in differentiating NSTEMI and USAP patients in the emergency department in the early stage.Materials and Methods: Eighty NSTEMI (Group 1) and 50 USAP (Group 2) patients who were consecutively admitted to our emergency department between November 2017 and May 2019 and diagnosed with acute coronary syndrome were included in the study. In both patient groups, PI measured with the Massimo-SET Root 7362A RDS7 non-invasive pulse oximetry probe and other routine laboratory measurements were measured and compared.Results: The mean PI was significantly lower in NSTEMI patients (p <0.001). At 30-day patient follow-up, the PI of the reduce was significantly lower (P <0.001). The area under the curve was significantly lower for PI in NSTEMI patients (area under the curve 0.313, p = 0.016). At 30-day patient follow-up, the level of PI was significantly lower in the died patients than the survived patients (P <0.001). The area under the curve was significantly lower for PI in NSTEMI patients (area under the curve 0.313, p = 0.016).Conclusions: Although our study shows that PI may be an early marker in the distinguish of NSTEMI and USAP patients and may be useful in predicting the mortality of these patients, more extensive studies will support our hypothesis.