Emergency department (ED) triage prioritizes patients based on urgency of care; however, little previous testing of triage tools in a live ED environment has been performed.To determine the agreement between a computer decision tool and memory-based triage.Consecutive patients presenting to a large, urban, tertiary care ED were assessed in the usual fashion and by a blinded study nurse using a computerized decision support tool. Triage score distribution and agreement between the two triage methods were reported. A random subset of patients was selected and reviewed by a blinded expert panel as a consensus standard.Over five weeks, 722 ED patients were assessed; complete data were available from 693 (96%) score pairs. Agreement between the two methods was poor (kappa = 0.202; 95% confidence interval [95% CI] = 0.150 to 0.254); however, agreement improved when using weighted kappa (0.360; 95% CI = 0.305 to 0.415) or "within one" level kappa (0.732; 95% CI = 0.644 to 0.821). When compared with the expert panel, the nurse triage scores showed lower agreement (0.263; 95% CI = 0.133 to 0.394) than the tool (kappa = 0.426; 95% CI = 0.289 to 0.564). There was a significant down-triaging of patients when patients were triaged without the computerized tool. Admission rates also differed between the triage systems.There was significant discrepancy by nurses using memory-based triage when compared with a computer tool. Triage decision support tools can mitigate this drift, which has administrative implications for EDs.