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Background and Objective: Triage is the most important and the first stage of patient’s management at the time of arrival to hospital emergency department. Emergency severity index (ESI) is a common triage system worldwide. This study was aimed to evaluate the accuracy of ESI in emergency department of Imam Hossein hospital in Tehran, Iran. Materials and Methods: In this descriptive study the result of patients’ triage based on ESI were gathered for all patients referred to emergency department of Imam Hossein Hospital from January to April 2011. A questioner was filled for each patient by the nurse and a emergency specialist independently. The l for the degree of agreement of triage between nurse and clinician was 81% (95% CI: 0.79-0.83). The sensivity of triage for step I, II, III, IV and V were 100%, 53.2%, 90.7%, 67.1% and 98% respectively. The specificity of triage for step I, II, III, IV and V were 99.8%, 97.5%, 93.7%, 98.3% and 94% respectively. There was a significant overlapping between the triage step and the patient clinical outcome. Conclusion: This study showed that five steps triage contain a high accuracy and estimation of patient outcomes.