The extent to which patients presenting to emergency departments (EDs) with minor conditions contribute to delays and crowding is controversial. To test this question, we study the effect of low-complexity ED patients on the waiting times of other patients. We obtained administrative records on all ED visits to Ontario hospitals from April 2002 to March 2003. For each ED, we determined the association between the number of new low-complexity patients (defined as ambulatory arrival, low-acuity triage level, and discharged) presenting in each 8-hour interval and the mean ED length of stay and time to first physician contact for medium- and high-complexity patients. Covariates were the number of new high- and medium-complexity patients, mean patient age, sex distribution, hospital teaching status, work shift, weekday/weekend, and total patient-hours. Autoregression modeling was used given correlation in the data. One thousand ninety-five consecutive 8-hour intervals at 110 EDs were analyzed; 4.1 million patient visits occurred, 50.8% of patients were women, and mean age was 38.4 years. Low-, medium-, and high-complexity patients represented 50.9%, 37.1%, and 12% of all patients, respectively. Mean (median) ED length of stay was 6.3 (4.7), 3.9 (2.8), and 2.2 (1.6) hours for high-, medium-, and low-complexity patients, respectively, and mean (median) time to first physician contact was 1.1 (0.7), 1.3 (0.9), and 1.1 (0.8) hours. In adjusted analyses, every 10 low-complexity patients arriving per 8 hours was associated with a 5.4-minute (95% confidence interval [CI] 4.2 to 6.0 minutes) increase in mean length of stay and a 2.1-minute (95% CI 1.8 to 2.4 minutes) increase in mean time to first physician contact for medium- and high-complexity patients. Results were similar regardless of ED volume and teaching status. Low-complexity ED patients are associated with a negligible increase in ED length of stay and time to first physician contact for other ED patients. Reducing the number of low-complexity ED patients is unlikely to reduce waiting times for other patients or lessen crowding.