PURPOSE: Research regarding the effect of trainees on emergency department (ED) operations has demonstrated mixed results. In this study, the authors evaluated the effect of trainees on ED length of stay (LOS), door to medical provider (DTMP) time, and door to disposition decision (DTDD) time while accounting for covariates known to influence ED efficiency and timeliness. METHOD: The authors used retrospective cohort data for ED visits to Maine Medical Center's mixed adult and pediatric ED for the calendar years 2005 through 2009. Each visit was coded indicating the type of provider conducting the visit (student-attending, resident-attending, midlevel provider, or attending group). Ordinary least squares regression analyses were performed to examine the relationships between provider groups and ED LOS, DTMP time, and DTDD time. Hierarchical regression models were constructed to control for the confounding effects of triage acuity, time of year, laboratory testing, radiographic testing, and patient characteristics. RESULTS: The analysis of 246,142 visits found significant intergroup differences across provider groups for each outcome (P < .001). Multiple regression modeling revealed that treatment by trainees was a significant predictor of longer LOS (medical students and residents), shorter DTMP time (residents), and longer DTDD time (medical students and residents), after controlling for covariates. CONCLUSIONS: Laboratory and radiographic testing accounted for a much larger proportion of variation in outcomes than did trainees. The small increases in LOS and DTDD time are balanced by the decrease in DTMP time and the intangible benefits of educating trainees.