Purpose: In hemodynamically unstable patients, tissue perfusion is decreased and various means of continuous observation are required. However, prior methods for observing hemodynamic instability are invasive and/or difficult for continual observation. The present study evaluated the usefulness of the perfusion index (PI) in monitoring hemodynamically unstable patients in the emergency department. Methods: From December, 2009 to April, 2010, patients admitted to our hospital emergency department with signs of hemodynamic instability were studied. Vital signs and stroke volume index (SVI) and cardiac index (CI) were measured, and PI was continuously monitored by a Radical-7 pulse oximeter (Masimo, USA). Each variable was measured 1 hour before and after treatment, and treatment methods included fluid therapy and vasoconstrictors, according to the patient condition. Status on changes and correlation between variables following treatment were confirmed through the Wilcoxon rank test and Spearman correlation test, respectively. The standard level of significance was p<0.05. Results: Of the 26 patients, 19 were male (73.1%), with an average age of 68.54±14.32 years. Significant changes in each variable 1 hour before and after treatment were mean arterial pressure 53.51±8.29mmHg / 74.68±12.95 mmHg (p<0.001), SVI 26.70±14.04/34.27±13.56(p<0.001), CI 2.12±1.02/3.12±1.53(p<0.001), and PI 1.23±1.00/1.77± 1.32(p<0.001). Variations of heart rate (R=-0.430, p=0.032) and SVI (R=0.432, p=0.031) were correlated to the variation of PI (dPI). Conclusion: PI may be useful in monitoring hemodynamically unstable patients in the ER.