Objective: Evaluate the impact of adding emergency medicine residents to a medium-size urban hospital by comparing emergency department (ED) admission rate, total census, length of stay (LOS), and proportion of patients who left without being seen (LWBS).Methods: Using the student t-test, the study compared commonly used ED metrics for a mid-sized urban hospital (annual census 43,000) for the four-month period prior to (March-June 2006) and after (March-June 2007) residents began providing 24-hour coverage at the institution.Results: There was no significant difference in the number of patients seen (NPS) in the two time periods, 14,471 and 14,699 patients respectively (p=0.507). Analysis of the NPS and LWBS was not statistically significant. The percentage of patients who LWBS decreased with the presence of residents (6.5% to 5.8%, p=0.531), and the overall ED LOS was similar (210 min vs. 219 min, p=0.56). Admission rate data demonstrated that residents had a similar admission rate (17.5% vs. 18%, p =0.332).Conclusion: ED flow depends on a number of variables with complex interactions. When comparing two similar time periods in consecutive years, the presence of resident physicians in the ED had no effect on the number of patients seen, patient LOS in the ED, or LWBS, thus supporting the conclusion that residents did not adversely affect the patient flow within the ED. [West J Emerg Med. 2010; 11(4):333-335.]