OBJECTIVES: The Emergency Severity Index (ESI) version 3 is a valid and reliable 5-level triage instrument that is gaining in popularity. A unique component of the ESI algorithm is prediction of resource consumption. Our objective was to validate the ESI version 3 triage algorithm in a clinical setting for the following outcome measures: actual resource consumption and patient length of stay in the emergency department and hospital. METHODS: We conducted a retrospective, descriptive study of 403 ED patients who presented to a large academic medical center. The following dependent variables were abstracted from the ED record: number of ED resources used and emergency department and hospital length of stay. The relationship between ESI level and each of the dependent variables was determined. RESULTS: Mean resource use decreased monotonically as a function of ESI level 1 (5), 2 (3.89), 3 (3.3), 4 (1.2) and 5 (0.2). The ED average length of stay (minutes) per ESI level was as follows: 1 (195), 2 (255), 3 (304), 4 (193), and 5 (98). ESI triage level did not predict hospital length of stay. CONCLUSIONS: The ESI algorithm accurately predicted ED resource intensity and gives administrators the opportunity to benchmark ED length of stay according to triage acuity level.