['We determine whether mandatory formal triage of walk-in emergency department (ED) patients provides timely recognition of the most acutely ill.', 'This retrospective cross-sectional study was conducted at a US urban academic ED, annual census 39,000, which uses Emergency Severity Index-5 triage (ESI-5) for all arriving patients. ESI-5 recommends that level 1 and 2 patients be treated by a physician immediately or within 10 minutes, respectively. For all high-acuity (ESI 1 or 2) patients presenting between January 1 and December 31, 2008, data from electronic medical records and registration and tracking systems were used to determine elapsed time from arrival to completion of triage (median, range, 95th percentile), proportion of these intervals that met ESI-5 recommendations, and whether triage throughput differed during peak arrival hours.', 'For 3,932 high-acuity walk-in visits (ESI 1=63; ESI 2=3,869), median time from arrival to triage completion was 12.3 minutes, range 0 to 128 minutes. Twenty-seven percent (95% confidence interval [CI] 26% to 29%) of high-acuity patients were taken to rooms on arrival; 41% (95% CI 40%, 43%), including those roomed immediately, completed triage within 10 minutes. Twenty-five percent (95% CI 24% to 26%) completed triage in greater than 20 minutes and 10% (95% CI 9% to 11%) greater than 30 minutes after arrival. Between 10 am and 10 pm (peak arrival hours), triage took longer for level 2 patients, and fewer met ESI recommendations.'] Does mandatory triage identify high-acuity patients within recommended time frames?