OBJECTIVES: Emergency Department [ED] overcrowding is a national problem.  This study evaluates the effect of a provider/nurse triage model (Rapid Evaluation Unit, REU) designed to improve the efficiency of the ED on ED patient volume and ambulance diversion hours. METHODS: Data on daily patient counts [inpatient, ED] and ambulance diversion hours for a single hospital in Baltimore were used to evaluate the impact of REU on hospital performance.  Data covered 2008, the year before the REU was implemented, and 2009 during which the REU was implemented incrementally.  The REU was operating on Monday-Wednesday from January 1st through March 31st, and then Monday through Friday from April 1st through December 31st.  Regression models were estimated for the daily counts for admissions, ED visits and ambulance diversion days controlling for time trend effects, day of the week and month of the year.  A spline functional form was used to test the effects of the REU on pre-2009 time trends and to improve model fit.  Models were adjusted for autocorrelation and heteroskedasticity as needed. RESULTS: The REU significantly increased ED patient visit counts by 5.5 visits per day [p<0.01] without increasing the number of hours the hospital was diverting ambulances due to over crowding [-0.83 hours per day, p>0.05]. For ED visits, all months exhibited significantly higher daily ED visit counts than in December, ranging from +9 to +28 visits per day.  The early work week [Monday-Wednesday] also exhibited significantly higher rates of ED visits than Saturday [range: 5-12 visits per day]. CONCLUSIONS: The REU was effective at increasing ED visits without increasing ambulance diversion hours.  We theorize that this may be due to a reduction in the time spent by patients waiting for treatment.