Background: To reduce bias in triage validity studies, one should focus on measures that can be obtained directly at triage. Objectives: To study the validity of Emergency Severity Index, Associations of the Emergency Severity Index triage categories with patients' vital signs at triage. Methods and study design: 260 patients were studied from March 2013 to June 2013.Each patient was triaged by the Researcher on duty and assigned a triage category using standard procedures. Immediately after the Researcher finished the triage assessment and reported the ESI triage category, the researcher registered patients’ gender, referrer, main complaint, age and measured vital signs that were measured by the triage nurse. The following vital signs were registered: blood pressure, pulse rate, respiratory rate, oxygen saturation, temperature, the AVPU score ( Alert, Voice, Pain and Unconsciousness), and pain, the data was analyzed using statistical designs and significant co relations between the study parameters were interpreted. Results and Conclusion: There is a significant correlation (p = 0.00) between the ESI scores and WPSS categories the findings of this study support the validity of the ESI as it showed that patients’ vital signs are associated with the ESI triage categories. There were strong associations between the ESI triage categories and patients’ WPSS scores at triage. However, no associations were found between pain scores and ESI triage categories, which indicates that ESI is a good triaging tool and reliability on pain scores should be revalidated.