Diagnostic delays in the emergency department (ED) are a serious patient safety concern. This retrospective cohort study included children treated at 954 EDs across 8 states, and examined the association between ED volume and delayed diagnosis of first-time diagnosis of an acute, serious conditions (e.g., bacterial meningitis, compartment syndrome, stroke). The researchers found that EDs with lower pediatric volume had higher rates of delayed diagnosis across 23 serious conditions.