An emergency department provides care for a full spectrum of undifferentiated disorders and its size may influence how well it can do so. Outcomes research in emergency medicine is limited, although outcomes in severe trauma and acute coronary syndromes have been shown to be influenced by available facilities and expertise. A department can be too small to provide the full spectrum of emergency care, as has been documented with General Practice Casualty Units. Some research suggests that a critical mass is required to ensure acceptable outcomes. Small departments may operate in a hierarchical system, led by teaching hospitals setting practice standards. However, these may be opinion as much as evidence based and not broadly applicable as a universal practice standard. Small departments can be overwhelmed by trauma and other major cases or when the teaching hospital is unable to accept emergency patients or transfers because they are operating at capacity. Further outcomes research and audit of individual hospitals is required to determine a minimum optimum size for emergency departments.