Emergency department crowding is a global challenge associated with adverse outcomes. It comprises distinct dimensions, including congestion and inpatient boarding, which may have different clinical implications but are often assessed using composite indices. We aimed to characterize patterns of crowding and evaluate the association between its key components and early in-hospital mortality in Brazil.

We conducted a multicenter observational study using operational data from hospitals participating in a national quality improvement program (2019–2025). Two indicators were analyzed: occupancy ratio (census divided by designated beds) and boarding proportion (admitted patients remaining in the ED divided by total census). Associations with early in-hospital mortality (<24 h from presentation) were assessed at the hospital-month level using adjusted negative binomial regression with admissions as an offset, adjusting for hospital characteristics and temporal factors.

A total of 160 hospitals were included. Crowding was persistent, with a median occupancy ratio of 1.03 and boarding proportion of 0.45. Boarding was associated with increased early mortality: each 0.10 increase in boarding proportion was associated with an IRR of 1.14 (95% CI 1.06–1.22). Occupancy was not associated with early mortality.

Boarding, but not occupancy, was associated with early mortality, suggesting that interventions targeting exit block may be more impactful than those focused solely on ED input or capacity. This study also provides a novel large-scale longitudinal description of ED crowding in Brazil.