The purpose of this paper is to examine the flow of patients through the emergency department of a large suburban acute general hospital and the factors that influence overcrowding and congestion. The paper broadly draws on the input-process-output systems paradigm as a basis for examining emergency department patient throughput. A data set of all attendances at the emergency department of a large suburban acute general hospital during the course of a full calendar year was examined using statistical tools. The analysis found that the major influencers of length of stay were patient-related factors such as age, severity of ailment (triage category), and whether or not the patient was admitted into the main hospital. Patients admitted into the hospital had significantly longer length of stay in emergency than those who were not admitted. The main influencers of patient arrivals were temporal factors such as time of day and day of arrival. These influencing factors are formed into a tentative model of emergency department congestion.