Background The factors associated with emergency ambulance re-attendance after being discharged at scene by paramedics are poorly understood.Method Retrospective observational study of consecutive patients attended but not transported to hospital by an emergency ambulance in Victoria, Australia between 01/07/2016 and 30/06/2017. Rates of re-attendance within 24 hours were examined using probabilistic matching. Adjusted logistic regression analyses were performed to determine factors associated with: 1) re-attendance; 2) re-attendances involving cardiac arrest or ‘lights/sirens’ transport to hospital.Results There were 97 061 non-transported patients of whom 3393 (3.5%) were re-attended within 24 hours. The outcome following initial attendance was: refused transport against paramedic advice (33.5%); referred to another healthcare provider (17.4%), and; discharged at scene (49.1%). On re-attendance, cardiac arrest occurred in 15 (0.5%) patients of which three were palliative, and 117 (3.4%) required ‘lights/sirens’ transport. The rate of cardiac arrest or ‘lights/sirens’ transport was higher in patients who initially refused transport against paramedic advice (4.8% vs. 3.4%, p=0.04). In the multivariable models, older age (Odds Ratio[OR]=1.02[1.01–1.02] per year), male gender (OR=1.25[1.15–1.35]), and an initial alcohol/drug-related issue (OR=1.50[1.05–2.14]) were associated with re-attendance within 24 hours. Older age (OR=1.02[1.01–1.03]), male gender (OR=1.69[1.12–2.55]), an initial pulse rate ≥120 pbm (OR=2.49[1.22–5.06]) and an initial respiratory rate >30 bpm (OR=3.42[1.43–8.14]) were independently associated with re-attendances involving cardiac arrest or ‘lights/sirens’ transport. Paramedic years of experience was not associated with re-attendance.Conclusion Ambulance re-attendance after discharge at scene is infrequent. Ambulance services may benefit from development of screening tools which identify patients at risk of harm after discharge at scene.Conflict of interest None.Funding None.