To examine the relationship between hospital and emergency department (ED) occupancy, as indicators of hospital overcrowding, and mortality after emergency admission. Retrospective analysis of 62 495 probabilistically linked emergency hospital admissions and death records. Three tertiary metropolitan hospitals between July 2000 and June 2003. All patients 18 years or older whose first ED attendance resulted in hospital admission during the study period. Deaths on days 2, 7 and 30 were evaluated against an Overcrowding Hazard Scale based on hospital and ED occupancy, after adjusting for age, diagnosis, referral source, urgency and mode of transport to hospital. There was a linear relationship between the Overcrowding Hazard Scale and deaths on Day 7 (r=0.98; 95% CI, 0.79-1.00). An Overcrowding Hazard Scale>2 was associated with an increased Day 2, Day 7 and Day 30 hazard ratio for death of 1.3 (95% CI, 1.1-1.6), 1.3 (95% CI, 1.2-1.5) and 1.2 (95% CI, 1.1-1.3), respectively. Deaths at 30 days associated with an Overcrowding Hazard Scale>2 compared with one of <3 were undifferentiated with respect to age, diagnosis, urgency, transport mode, referral source or hospital length of stay, but had longer ED durations of stay (risk ratio per hour of ED stay, 1.1; 95% CI, 1.1-1.1; P<0.001) and longer physician waiting times (risk ratio per hour of ED wait, 1.2; 95% CI, 1.1-1.3; P=0.01). Hospital and ED overcrowding is associated with increased mortality. The Overcrowding Hazard Scale may be used to assess the hazard associated with hospital and ED overcrowding. Reducing overcrowding may improve outcomes for patients requiring emergency hospital admission.