OBJECTIVE: To determine whether after-hours presentation to EDs is associated with differences in 7-day and 30-day mortality. The influence of patient case-mix and workforce staffing differences are also explored. METHODS: We conducted a retrospective observational study of 3.7 million ED episodes across 30 public hospitals in Queensland, Australia during May 2013-September 2015 using routinely collected hospital data linked to hospital staffing data and the death registry. Episodes were categorised as within/after-hours using time of presentation. Staffing was derived from payroll records and explored by defining 11 staffing ratios. RESULTS: Weekend presentation was slightly more associated (7-day mortality odds ratio 1.05, 95% confidence interval [CI] 1.01-1.10) or no more associated (30-day mortality odds ratio 1.01, 95% CI 0.98-1.03) with death than weekday presentation. When weeknights are included in the 'after-hours' period, odds ratios are smaller, so that after-hours presentation is no more associated (7-day mortality odds ratio 1.03, 95% CI 0.99-1.08) or less associated (30-day mortality odds ratio 0.95, 95% CI 0.93-0.97) with death. No significant after-hours patient case-mix differences were observed between weekday and weekend presentations for 7-day mortality. In other combinations of outcome and after-hours definition, some differences (especially measures relating to severity of presenting condition) were found. Staffing ratios were not strongly associated with any within/after-hours differences in ED mortality. CONCLUSIONS: After-hours presentation on the weekend to an ED is associated with higher 7-day mortality even after controlling for case-mix.