Objective: Uncontrolled emergency department (ED) visits lead to crowding and increase the time patients are in the ED. Following the 2015 Middle East respiratory syndrome (MERS) outbreak in Korea, EDs began to regulate visitors. The purpose of this study was to examine the results of regulating ED visits and to identify factors associated with increased ED visits. Methods: We conducted a cross-sectional observational study using patient information recorded in a visitor's book during the MERS outbreak from July 6 to July 26, 2015. The visitor's book included the patient's and visitor's names, their relationship, and the day and time of the visit. The collected patient information included sex, age, mode of visit, length of stay in the ED, and disposition. The risk ratio and 95% confidence interval for each variable were calculated using Poisson regression. The primary outcome of this study was the number of visitors per patient according to their characteristics. Results: Overall, 4,139 patients and 5,642 visitors were registered during the study period. More people visited during weekdays and office hours. The characteristics of patients that increased visitors per patient were when the mode of visit was a public ambulance and when the length of stay in the ED was longer than 12 hours. Conclusion: During the study period, ED visits were strongly controlled by a government guidance, but there was still a significant number of visitors. If we do not restrict visitors, infection control in the ED will be difficult. It is urgent to develop policies and cultural improvement to control ED visits.